Rustbuild: enable -Zsplit-metadata for stage != 0

Author: bjorn3

src/bootstrap/src/core/builder.rs

@@ -1484,6 +1484,12 @@ impl<'a> Builder<'a> {
         hostflags.arg("-Zunstable-options");
         hostflags.arg("--check-cfg=cfg(bootstrap)");
 
+        // cfg(bootstrap) unconditionally pass this once the bootstrap compiler understands it
+        if stage != 0 {
+            // FIXME remove once cargo enables this by default
+            rustflags.arg("-Zsplit-metadata");
+        }
+
         // FIXME: It might be better to use the same value for both `RUSTFLAGS` and `RUSTDOCFLAGS`,
         // but this breaks CI. At the very least, stage0 `rustdoc` needs `--cfg bootstrap`. See
         // #71458.