Restrict linker version script of proc-macro crates to just its two symbols Restrict linker version script of proc-macro crates to just the two symbols of each proc-macro crate. The main known effect of doing this is to stop including `#[no_mangle]` symbols in the linker version script. Background: The combination of a proc-macro crate with an import of another crate that itself exports a no_mangle function was broken for a period of time, because: * In PR #99944 we stopped exporting no_mangle symbols from proc-macro crates; proc-macro crates have a very limited interface and are meant to be treated as a blackbox to everything except rustc itself. However: he constructed linker version script still referred to them, but resolving that discrepancy was left as a FIXME in the code, tagged with issue #99978. * In PR #108017 we started telling the linker to check (via the`--no-undefined-version` linker invocation flag) that every symbol referenced in the "linker version script" is provided as linker input. So the unresolved discrepancy from #99978 started surfacing as a compile-time error (e.g. #111888). Fix #111888 Fix #99978.