nolibc: implement poll() based on ppoll()

Some architectures like arm64 do not implement poll() and have to use
ppoll() instead.

nolibc.h

@@ -1671,7 +1671,17 @@ int sys_pivot_root(const char *new, const char *old)
 static __attribute__((unused))
 int sys_poll(struct pollfd *fds, int nfds, int timeout)
 {
+#if defined(__NR_ppoll)
+	struct timespec t;
+
+	if (timeout >= 0) {
+		t.tv_sec  = timeout / 1000;
+		t.tv_nsec = (timeout % 1000) * 1000000;
+	}
+	return my_syscall4(__NR_ppoll, fds, nfds, (timeout >= 0) ? &t : NULL, NULL);
+#else
 	return my_syscall3(__NR_poll, fds, nfds, timeout);
+#endif
 }
 
 static __attribute__((unused))