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/Tests/Kernel/invalid-path-pointer-assert.cpp

@@ -24,10 +24,10 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <Kernel/API/Syscall.h>
 #include <errno.h>
 #include <stdio.h>
 #include <sys/stat.h>
+#include <syscall.h>
 
 int main()
 {