make epoll_[p]wait a cancellation point

this is a Linux-specific function and not covered by POSIX's
requirements for which interfaces are cancellation points, but glibc
makes it one and existing software relies on it being one.

at some point a review for similar functions that should be made
cancellation points should be done.

src/linux/epoll.c

@@ -24,9 +24,9 @@ int epoll_ctl(int fd, int op, int fd2, struct epoll_event *ev)
 
 int epoll_pwait(int fd, struct epoll_event *ev, int cnt, int to, const sigset_t *sigs)
 {
-	int r = __syscall(SYS_epoll_pwait, fd, ev, cnt, to, sigs, _NSIG/8);
+	int r = __syscall_cp(SYS_epoll_pwait, fd, ev, cnt, to, sigs, _NSIG/8);
 #ifdef SYS_epoll_wait
-	if (r==-ENOSYS && !sigs) r = __syscall(SYS_epoll_wait, fd, ev, cnt, to);
+	if (r==-ENOSYS && !sigs) r = __syscall_cp(SYS_epoll_wait, fd, ev, cnt, to);
 #endif
 	return __syscall_ret(r);
 }