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

[Christopher Di Bella via llvm-commits]

https://github.com/cjdb requested changes to this pull request.

Thanks for working on this!

IIRC it was @compnerd who convinced me that llvm-libgcc should **not** be compatible with an independent compiler-rt and libunwind, and that was because you only need one or the other. I still agree with that since llvm-libgcc is only intended for toolchain vendors, and not for casual use.

It would be good to get some exposition on why this PR changes this, and input from @compnerd regarding that design decision too.

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