[llvm] [libc] add multi-platform pre-commit github actions (PR #119104)

[Schrodinger ZHU Yifan via llvm-commits]

SchrodingerZhu wrote:

The windows/macos testing are all light weight, even without caching, it finishes within 10 mins. The Linux one is a bit more problematic, as there are 9000+ testing units. But all of these finishes in 20mins.

https://github.com/llvm/llvm-project/pull/119104