tsan: fix epoll_pwait2 interceptor

epoll_pwait2 is new and may not be present in libc and/or kernel.
Since we effectively add it to libc (as will be probed by the program
using dlsym or a weak function pointer) we need to handle the case
when it's not present in the actual libc.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D138929