Add wrapper macros for "final" and "override"

Add wrapper macros for "final" and "override"

Summary:
... which are supported in gcc 4.7, but not 4.6, and they enable useful
optimizations.  As long as we're planning to support both 4.6 and 4.7 in
our code base, we should write code using these macros.

Test Plan:
test added, compiled with both 4.6 and 4.7, verified the
assertion in the test comment by looking at dissassembly output in the
4.7 version

Reviewed By: delong.j@fb.com

FB internal diff: D520343