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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
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.