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

[via llvm-commits]

ur4t wrote:

> This patch makes `libgcc` buildable but note that `llvm-libgcc` is not a target of the build directory

@MaskRay This issue is fixed in [c790fd0](https://github.com/llvm/llvm-project/pull/65455/commits/c790fd05b3cd4a5c5cbaff1d9855a8c4d7a1e2cb).

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