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

[Louis Dionne via llvm-commits]

ldionne wrote:

Are we waiting on anything to merge this? I think folks' expectations of how patches are merged might be unclear right now due to the Github PR transition. With Phabricator the patch author would push the change and the "PR" would be closed automatically. Now with Github PRs someone has to press "squash and merge" but it might not be clear whose responsibility it is to do this.

@cjdb @MaskRay We're ready to land this, right? If so, I think either of you can "Squash and merge" (or @ur4t if they are able to).

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