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.

Kernel/API/Syscall.h

@@ -504,7 +504,6 @@ inline uintptr_t invoke(Function function, T1 arg1, T2 arg2, T3 arg3)
 #define __ENUMERATE_SYSCALL(x) using Syscall::SC_##x;
 ENUMERATE_SYSCALLS(__ENUMERATE_SYSCALL)
 #undef __ENUMERATE_SYSCALL
-#define syscall Syscall::invoke
 
 }