LibC: Implement pthread cancelation

This code is not tested at all but it looks correct xD

userspace/libraries/LibC/poll.cpp

@@ -1,9 +1,11 @@
 #include <poll.h>
+#include <pthread.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
 int poll(struct pollfd fds[], nfds_t nfds, int timeout)
 {
+	pthread_testcancel();
 	if (timeout < 0)
 		return ppoll(fds, nfds, nullptr, nullptr);
 	const timespec timeout_ts {
@@ -15,5 +17,6 @@ int poll(struct pollfd fds[], nfds_t nfds, int timeout)
 
 int ppoll(struct pollfd fds[], nfds_t nfds, const struct timespec* timeout, const sigset_t* sigmask)
 {
+	pthread_testcancel();
 	return syscall(SYS_PPOLL, fds, nfds, timeout, sigmask);
 }