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/LibSystem/CMakeLists.txt

@@ -0,0 +1,7 @@
+set(SOURCES
+    syscall.cpp
+)
+
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -nostdlib")
+serenity_libc(LibSystem system)
+target_include_directories(LibSystem PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})