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/LibPthread/CMakeLists.txt

@@ -4,5 +4,5 @@ set(SOURCES
 )
 
 serenity_libc(LibPthread pthread)
-target_link_libraries(LibPthread LibC)
+target_link_libraries(LibPthread LibC LibSystem)
 target_include_directories(LibPthread PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})