HLTrigger/Timer/test: add a thread-based cpu clock for OS X

Add a clock to measure the cpu time used by the current thread, with microsecond
granularity, based on thread_info(mach_thread_self(), THREAD_BASIC_INFO, ...).
Merge pull request #4623 from fwyzard/mach_thread_info_clock

HLTrigger/Timer/test: add a thread-based cpu clock for OS X