Reland "Explicitly include compiler-rt builtins library in linking"

This is a reland of commit 31b9cb8

Original change's description:
> Explicitly include compiler-rt builtins library in linking
>
> This makes the intrinsic symbols contained within considered as strong
> symbols, so that when the Rust stdlib intrinsics are marked as weak,
> the C++ ones will take precedence. Otherwise, even if they are marked
> weak, the Rust stdlib intrinsics are selected by the linker.
>
> It may be the Rust intrinsics are preferable, but we want to stay on
> the C++ intrinsics (for C++ and Rust code in mixed binaries) unless we
> can demonstrate a good reason to switch.
>
> Upstream bug for marking Rust intrinsic symbols weak:
> rust-lang/compiler-builtins#525
>
> To test and verify this works:
> 1. Revert the changes to shift.rs in rust-lang/compiler-builtins@1634193 to third_party/rust-toolchain/lib/rustlib/src/rust/vendor/compiler_builtins-*/src/int/shift.rs
> 2. Apply https://chromium-review.googlesource.com/c/chromium/src/+/45461823
> 3. Follow the instructions in its description to build and run bad_intrinsics
> 4. The bad_intrinsics binary panics without this CL, but not with this CL, meaning the C++ intrinsics are used.
>
> Bug: 1445978
> Change-Id: Ib251660346d03902f531285999aeeabc28882049
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4546164
> Commit-Queue: Arthur Eubanks <[email protected]>
> Reviewed-by: Arthur Eubanks <[email protected]>
> Auto-Submit: danakj <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#1150647}

Bug: 1445978
Change-Id: I04302cd1f67b911c81b3413c09d391379380710b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4574784
Commit-Queue: danakj <[email protected]>
Reviewed-by: Arthur Eubanks <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1151259}

Author: danakj

build/config/compiler/BUILD.gn

@@ -1554,6 +1554,96 @@ config("compiler_arm") {
   }
 }
 
+# Rust and C++ both provide intrinsics for LLVM to call for math operations. We
+# want to use the C++ intrinsics, not the ones in the Rust compiler_builtins
+# library. The Rust symbols are marked as weak, so that they can be replaced by
+# the C++ symbols. This config ensures the C++ symbols exist and are strong in
+# order to cause that replacement to occur by explicitly linking in clang's
+# compiler-rt library.
+config("compiler_builtins") {
+  visibility = [ ":runtime_library" ]
+
+  _supported = true
+  _dir = ""
+  _libname = "builtins"
+  _suffix = ""
+  _ext = "a"
+  if (!toolchain_has_rust) {
+    # Since there's no Rust in the toolchain, there's no concern that we'll use
+    # the Rust stdlib's intrinsics here.
+    _supported = false
+  } else if (is_win) {
+    _dir = "windows"
+    _ext = "lib"
+    if (current_cpu == "x64") {
+      _suffix = "-x86_64"
+    } else if (current_cpu == "x86") {
+      _suffix = "-i386"
+    } else {
+      assert(false)  # Unhandled cpu type
+    }
+  } else if (is_mac) {
+    _dir = "darwin"
+    _libname = "osx"
+  } else if (is_ios) {
+    import("//build/config/ios/config.gni")  # For `target_environment`
+
+    _dir = "darwin"
+    if (target_environment == "simulator") {
+      _libname = "iossim"
+    } else {
+      _libname = "ios"
+    }
+  } else if (is_linux || is_chromeos) {
+    if (current_cpu == "x64") {
+      _dir = "x86_64-unknown-linux-gnu"
+    } else if (current_cpu == "x86") {
+      _dir = "i386-unknown-linux-gnu"
+    } else if (current_cpu == "arm") {
+      _dir = "armv7-unknown-linux-gnueabihf"
+    } else if (current_cpu == "arm64") {
+      _dir = "aarch64-unknown-linux-gnu"
+    } else {
+      assert(false)  # Unhandled cpu type
+    }
+  } else if (is_fuchsia) {
+    if (current_cpu == "x64") {
+      _dir = "x86_64-unknown-fuchsia"
+    } else if (current_cpu == "arm64") {
+      _dir = "aarch64-unknown-fuchsia"
+    } else {
+      assert(false)  # Unhandled cpu type
+    }
+  } else if (is_android) {
+    _dir = "linux"
+    if (current_cpu == "x64") {
+      _suffix = "-x86_64-android"
+    } else if (current_cpu == "x86") {
+      _suffix = "-i686-android"
+    } else if (current_cpu == "arm") {
+      _suffix = "-arm-android"
+    } else if (current_cpu == "arm64") {
+      _suffix = "-aarch64-android"
+    } else {
+      assert(false)  # Unhandled cpu type
+    }
+  } else {
+    assert(false)  # Unhandled target platform
+  }
+
+  if (_supported) {
+    _clang_lib_dir = "$clang_base_path/lib/clang/$clang_version/lib"
+    libs = [ "$_clang_lib_dir/$_dir/libclang_rt.$_libname$_suffix.$_ext" ]
+  } else {
+    not_needed([
+                 "_dir",
+                 "_libname",
+                 "_suffix",
+                 "_ext",
+               ])
+  }
+}
+
 # runtime_library -------------------------------------------------------------
 #
 # Sets the runtime library and associated options.
@@ -1575,6 +1665,10 @@ config("runtime_library") {
     configs += [ "//build/config/c++:runtime_library" ]
   }
 
+  if (toolchain_has_rust) {
+    configs += [ ":compiler_builtins" ]
+  }
+
   # TODO(crbug.com/830987): Come up with a better name for is POSIX + Fuchsia
   # configuration.
   if (is_posix || is_fuchsia) {