LibCore+LibSystem: Move syscall wrappers from LibSystem to LibCore

With this change, System::foo() becomes Core::System::foo().

Since LibCore builds on other systems than SerenityOS, we now have to
make sure that wrappers work with just a standard C library underneath.

Userland/Libraries/LibCore/CMakeLists.txt

@@ -27,6 +27,7 @@ set(SOURCES
     SecretString.cpp
     Socket.cpp
     StandardPaths.cpp
+    System.cpp
     TCPServer.cpp
     TCPSocket.cpp
     Timer.cpp