[llvm] [llvm-libgcc][CMake] Refactor llvm-libgcc (PR #65455)

[via llvm-commits]

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ur4t wrote:

In the refactored approach, we modify `libunwind.so` added in `libunwind` directly and no custom shared library is required, so the dummy source file can be deleted.

https://github.com/llvm/llvm-project/pull/65455