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

[Louis Dionne via llvm-commits]

ldionne wrote:

> 
> 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.

FWIW that may not always be possible for folks who don't have direct commit access to the repo. But feel free to handle that however makes the most sense to you. For libc++/libc++abi, I often merge on behalf of folks to make sure that the PR isn't forgotten, etc.

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