[llvm] [libc] [reland][libc] add epoll_wait functions (PR #79635)

[via llvm-commits]

michaelrj-google wrote:

The overlay build was broken by a change I'd made to better match the style for type includes. I've fixed the problem and it builds clean in both overlay and fullbuild now.

https://github.com/llvm/llvm-project/pull/79635