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/Utilities/crash.cpp

@@ -27,13 +27,13 @@
 
 #include <AK/Function.h>
 #include <AK/String.h>
-#include <Kernel/API/Syscall.h>
 #include <Kernel/IO.h>
 #include <LibCore/ArgsParser.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/mman.h>
 #include <sys/wait.h>
+#include <syscall.h>
 
 #pragma GCC optimize("O0")