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

[Fangrui Song via llvm-commits]

MaskRay wrote:

I agree with a better description is needed.

> 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.

I feel the author is the best one to perform the action, as they can adjust the description to be clearer, rebase as needed, or do other minor adjustments that do not need re-review.

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