linux-user: Use safe_syscall wrapper for epoll_wait syscalls

Use the safe_syscall wrapper for epoll_wait and epoll_pwait syscalls.

Since we now directly use the host epoll_pwait syscall for both
epoll_wait and epoll_pwait, we don't need the configure machinery
to check whether glibc supports epoll_pwait(). (The kernel has
supported the syscall since 2.6.19 so we can assume it's always there.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
