Use modern C++ attributes instead of __attribute__ voodoo.

This is quite nice, although I wish [[gnu::always_inline]] implied inline.
Also "gnu::" is kind of a wart, but whatcha gonna do.

Kernel/VirtualFileSystem.h

@@ -54,7 +54,7 @@ public:
         RetainPtr<FS> m_guest_fs;
     };
 
-    static VFS& the() PURE;
+    [[gnu::pure]] static VFS& the();
 
     VFS();
     ~VFS();