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/DevTools/UserspaceEmulator/Emulator.cpp

@@ -32,7 +32,6 @@
 #include <AK/Format.h>
 #include <AK/LexicalPath.h>
 #include <AK/MappedFile.h>
-#include <Kernel/API/Syscall.h>
 #include <LibELF/AuxiliaryVector.h>
 #include <LibELF/Image.h>
 #include <LibELF/Validation.h>
@@ -53,6 +52,7 @@
 #include <sys/stat.h>
 #include <sys/time.h>
 #include <sys/uio.h>
+#include <syscall.h>
 #include <termios.h>
 #include <unistd.h>