tsan: intercept epoll_pwait2

It's a new syscall similar to epoll_pwait.
Add a similar interceptor for it and add synchronization
annotations in epoll_wait* syscall wrappers.
Testing this is problematic b/c it's not present in glibc
and the syscall itself may not be supported by the kernel.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D138574