Add epoll_pwait()

(akpm: stolen from Andi's queue, because I want to send the signalfd patches
which also add syscalls.  Not sure what the __IGNORE_getcpu is for).
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>