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

[via llvm-commits]

ur4t wrote:

@MaskRay It seems that Github enforce a column limit of 70 characters in "Squash and Merge" commit messages ([refined-github#1475](https://github.com/refined-github/refined-github/issues/1475)). Commit messages targeting a 80 character limitation will be truncated again when texts are copied and pasted directly with original line breakages (7ac1f60f01a73f9678bba2385e933864ecce8622 and af935cf0ee8a26dcf0860ed0c72d74b4a1cb1314). Should we mention this behaviour in documents for the Github PR transition?

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