Don't need to call stop_hpc from emulate_async_signal.
Rework perfcounter interface into a C++ class.

Starts the process of renaming rbc/rcb to 'ticks'.

Stops disabling counters before we close the fd. That's unnecessary
overhead.

Instead of using an #ifdef to guard the extra perf counters,
use a static bool function instead. #ifdefs are evil.
Rename hpc.h/.cc to PerfCounters.h/.cc.