[libc] [llvm] [libc] add remaining epoll functions, pipe (PR #84587)

[Nick Desaulniers via llvm-commits]

https://github.com/nickdesaulniers commented:

In the future, consider breaking these function implementations up into distinct PRs.  That way, we can make incremental progress on landing pieces.  Things flagged in code review don't block landing unrelated functions.

https://github.com/llvm/llvm-project/pull/84587