linux: try epoll_pwait if epoll_wait is missing
linux: try epoll_pwait if epoll_wait is missing

It seems that epoll_wait is implemented in glibc in terms of epoll_pwait and
new architectures (like arm64) do not implement the epoll_wait syscall at all.
So if epoll_wait errors with ENOSYS, just call epoll_pwait.

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