Userland: Add LibSystem and funnel all syscalls through it

This achieves two things:

- Programs can now intentionally perform arbitrary syscalls by calling
  syscall(). This allows us to work on things like syscall fuzzing.

- It restricts the ability of userspace to make syscalls to a single
  4KB page of code. In order to call the kernel directly, an attacker
  must now locate this page and call through it.

Userland/Libraries/LibPthread/pthread.cpp

@@ -28,7 +28,6 @@
 #include <AK/Atomic.h>
 #include <AK/Debug.h>
 #include <AK/StdLibExtras.h>
-#include <Kernel/API/Syscall.h>
 #include <limits.h>
 #include <pthread.h>
 #include <serenity.h>
@@ -36,6 +35,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <syscall.h>
 #include <time.h>
 #include <unistd.h>