speed up threaded fork

after fork, we have a new process and the pid is equal to the tid of
the new main thread. there is no need to make two separate syscalls to
obtain the same number.

src/process/fork.c

@@ -10,8 +10,7 @@ pid_t fork(void)
 	ret = syscall(SYS_fork);
 	if (libc.lock && !ret) {
 		pthread_t self = __pthread_self();
-		self->pid = syscall(SYS_getpid);
-		self->tid = syscall(SYS_gettid);
+		self->tid = self->pid = syscall(SYS_getpid);
 		libc.threads_minus_1 = 0;
 	}
 	if (libc.fork_handler) libc.fork_handler(!ret);