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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> [MaskRay](https://github.com/MaskRay) approved these changes on behalf of https://github.com/orgs/llvm/teams/pr-subscribers-libunwind [now](https://github.com/llvm/llvm-project/pull/65455#pullrequestreview-1615864037)

This removed llvm-libunwind from the reviewer list. I wish that somebody familiar with CMake may take a look.

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