Auto merge of rust-lang#105007 - dlaugt:solaris-fs-link, r=ChrisDenton

linkat() not available in the system headers of Solaris 10

I've installed rustup on x86_64-unknown-linux-gnu and would like to use the target sparcv9-sun-solaris. For this, I have built a gcc from the source code for cross-compiling to sparcv9-sun-solaris2.10 with system headers of Solaris 10.

With the following hello word example:
main.rs:
```rust
fn main() {
    println!("Hello, world!");
}
```
I had a compilation error:
```
$ rustc -v --target sparcv9-sun-solaris -C linker=/opt/cross-solaris/gcc730/bin/sparcv9-sun-solaris2.10-gcc main.rs
error: linking with `/opt/cross-solaris/gcc730/bin/sparcv9-sun-solaris2.10-gcc` failed: exit status: 1
  |
  = note: "/opt/cross-solaris/gcc730/bin/sparcv9-sun-solaris2.10-gcc" "-m64" "/tmp/rustcgebYgj/symbols.o" "main.main.89363361-cgu.0.rcgu.o" "main.main.89363361-cgu.1.rcgu.o" "main.main.89363361-cgu.2.rcgu.o" "main.main.89363361-cgu.3.rcgu.o" "main.main.89363361-cgu.4.rcgu.o" "main.main.89363361-cgu.5.rcgu.o" "main.csypsau9u2r8348.rcgu.o" "-Wl,-z,ignore" "-L" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib" "-Wl,-Bstatic" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libstd-fa47c8247d587714.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libpanic_unwind-5c87bbe223e6c2a3.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libobject-d484934062ff9fbb.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libmemchr-e8dbd5835abcbf43.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libaddr2line-909ad09329bde2f9.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libgimli-4d74a3be929697ac.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/librustc_demangle-47cbe1d7f7271ae1.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libstd_detect-239fd2d25fb32a00.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libhashbrown-c4a7ce45fb9dec19.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libminiz_oxide-fa6bc3d9bfb4e402.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libadler-419f5a82ddd339a3.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/librustc_std_workspace_alloc-7672b378962c11be.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libunwind-0f9e07f0a032c000.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libcfg_if-ede7757c356dfb28.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/liblibc-808d56fbc668148a.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/liballoc-784767fe059ad3fe.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/librustc_std_workspace_core-aa31d7ef0556bbe1.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libcore-81d07df07db18847.rlib" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libcompiler_builtins-313a510e63006db2.rlib" "-Wl,-Bdynamic" "-lsocket" "-lposix4" "-lpthread" "-lresolv" "-lgcc_s" "-lc" "-lm" "-lrt" "-lpthread" "-lsendfile" "-llgrp" "-L" "/home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib" "-o" "main" "-nodefaultlibs"
  = note: /opt/cross-solaris/gcc730/lib/gcc/sparcv9-sun-solaris2.10/7.3.0/../../../../sparcv9-sun-solaris2.10/bin/ld: warning: -z ignore ignored.
          /home/dlaugt/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/sparcv9-sun-solaris/lib/libstd-fa47c8247d587714.rlib(std-fa47c8247d587714.std.5c42d2c1-cgu.0.rcgu.o): In function `std::sys::unix::fs::link::h3683dfbfbb4995cb':
          /rustc/897e37553bba8b42751c67658967889d11ecd120/library/std/src/sys/unix/fs.rs:1407: undefined reference to `linkat'
          collect2: error: ld returned 1 exit status

  = help: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link
  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorustc-link-libkindname)
```

linkat() is not available in the system headers of Solaris 10. The hello word example works fine when I build/use rust with this PR change.

Author: bors

library/std/src/sys/unix/fs.rs

@@ -34,7 +34,7 @@ use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
     target_os = "watchos",
 ))]
 use crate::sys::weak::syscall;
-#[cfg(any(target_os = "android", target_os = "macos"))]
+#[cfg(any(target_os = "android", target_os = "macos", target_os = "solaris"))]
 use crate::sys::weak::weak;
 
 use libc::{c_int, mode_t};
@@ -43,6 +43,7 @@ use libc::{c_int, mode_t};
     target_os = "macos",
     target_os = "ios",
     target_os = "watchos",
+    target_os = "solaris",
     all(target_os = "linux", target_env = "gnu")
 ))]
 use libc::c_char;
@@ -1497,8 +1498,8 @@ pub fn link(original: &Path, link: &Path) -> io::Result<()> {
                     // Android has `linkat` on newer versions, but we happen to know `link`
                     // always has the correct behavior, so it's here as well.
                     cvt(unsafe { libc::link(original.as_ptr(), link.as_ptr()) })?;
-                } else if #[cfg(target_os = "macos")] {
-                    // On MacOS, older versions (<=10.9) lack support for linkat while newer
+                } else if #[cfg(any(target_os = "macos", target_os = "solaris"))] {
+                    // MacOS (<=10.9) and Solaris 10 lack support for linkat while newer
                     // versions have it. We want to use linkat if it is available, so we use weak!
                     // to check. `linkat` is preferable to `link` because it gives us a flag to
                     // specify how symlinks should be handled. We pass 0 as the flags argument,