Add `--no-undefined-version` link flag and fix associated breakage
LLVM upstream sets `--no-undefined-version` by default in lld: https://reviews.llvm.org/D135402.
Due to a bug in how version scripts are generated, this breaks the `dylib` output type for most crates. See https://github.com/rust-lang/rust/issues/105967#issuecomment-1428671533 for details.
This PR adds the flag to gcc flavor linkers in anticipation of this LLVM change rolling in, and patches `rustc` to not attempt to export `__rust_*` allocator symbols when they weren't generated.
Fixes#105967
342ca46517