Make #[cfg(bootstrap)] not error in proc macros on later stages

As was discovered in #93628 (comment) ,
adding #[cfg(bootstrap)] to a rust-internal proc macro crate
would yield an unexpected cfg name error, at least on later
stages wher the bootstrap cfg arg wasn't set.

rustc already passes arguments to mark bootstrap as expected,
however the means of delivery through the RUSTFLAGS env var
is unable to reach proc macro crates, as described
in the issue linked in the code this commit touches.

This wouldn't be an issue for cfg args that get passed through
RUSTFLAGS, as they would never become *active* either, so
any usage of one of these flags in a proc macro's code would
legitimately yield a lint warning. But since dc30258,
rust takes extra measures to pass --cfg=bootstrap even in
proc macros, by passing it via the wrapper. Thus, we need
to send the flags to mark bootstrap as expected also from the
wrapper, so that #[cfg(bootstrap)] also works from proc macros.

I want to thank Urgau and jplatte for helping me find the cause of this. ❤️

src/bootstrap/bin/rustc.rs

@@ -127,12 +127,18 @@ fn main() {
             }
         }
 
+        // Cargo doesn't pass RUSTFLAGS to proc_macros:
+        // https://github.com/rust-lang/cargo/issues/4423
+        // Thus, if we are on stage 0, we explicitly set `--cfg=bootstrap`.
+        // We also declare that the flag is expected, which is mainly needed for
+        // later stages so that they don't warn about #[cfg(bootstrap)],
+        // but enabling it for stage 0 too lets any warnings, if they occur,
+        // occur more early on, e.g. about #[cfg(bootstrap = "foo")].
         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");
         }
+        cmd.arg("-Zunstable-options");
+        cmd.arg("--check-cfg=values(bootstrap)");
     }
 
     if let Ok(map) = env::var("RUSTC_DEBUGINFO_MAP") {