Pass --cfg=bootstrap for proc_macros or build scripts built by stage0

Cargo ignores RUSTFLAGS when building proc macro crates. However,
sometimes rustc_macro needs to have conditional compilation when there
are breaking changes to the `libproc_macro` API (see for example
tell the difference between stage 0 and stage 1.

Another alternative is to unconditionally build rustc_macros with the
master libstd instead of the beta one (i.e. use `--sysroot
stage0-sysroot`), but that led to strange and maddening errors:

```
error[E0460]: found possibly newer version of crate `std` which `proc_macro2` depends on
  --> /home/joshua/.local/lib/cargo/registry/src/jiasu.xzqcsaa.nyc.mn-1ecc6299db9ec823/tracing-attributes-0.1.13/src/lib.rs:90:5
   |
90 | use proc_macro2::TokenStream;
   |     ^^^^^^^^^^^
   |
   = note: perhaps that crate needs to be recompiled?
   = note: the following crate versions were found:
           crate `std`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-b3602c301b71cc3d.rmeta
           crate `proc_macro2`: /home/joshua/rustc2/build/x86_64-unknown-linux-gnu/stage0-rustc/release/deps/libproc_macro2-a83c1f01610c129e.rlib
```

Author: jyn514

src/bootstrap/bin/rustc.rs

@@ -124,6 +124,13 @@ fn main() {
                 cmd.arg("-C").arg("target-feature=-crt-static");
             }
         }
+
+        if stage == "0" {
+            // Cargo doesn't pass RUSTFLAGS to proc_macros:
+            // https://github.com/rust-lang/cargo/issues/4423
+            // Set `--cfg=bootstrap` explicitly instead.
+            cmd.arg("--cfg=bootstrap");
+        }
     }
 
     if let Ok(map) = env::var("RUSTC_DEBUGINFO_MAP") {