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

[Christopher Di Bella via llvm-commits]

cjdb wrote:

@ur4t before I squash and merge, could you please provide me with a commit message that explains why the changes were necessary (as a comment)? I've looked at all the commit messages and text, but I don't see anything that captures the full message. If you've already written something, a link to that will be fine :)

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