gbuild: Framework for performance unit tests using callgrind

Run them with "make perfcheck" (toplevel or in a module)

To add a new performance test:
  - Add it as a perfcheck target
  - Use gb_CppunitTest_set_performance_test
  - Use CALLGRIND_ macros from valgrind/callgrind.h
    http://valgrind.org/docs/manual/cl-manual.html#cl-manual.clientrequests

Change-Id: I67c776dbe4db0a686607efeee7a7e4f3aeae0e5c