This provides samples to the statistical profiler introduced in D1214 based on heap allocation and blackhole-blocking events.
Additional sampling strategies (e.g. based on time, CPU performance counters,
etc.) are left for future work.
The implementation of heap sampling is admittedly a bit hacky.
Based on Peter Wortmann's c01384a26d7c9d22d26a760470bdb6379a2913ee.
Lacking a better idea, I follow Peter's lead and lay claim to R9
to get the attribution address to stg_gc_noregs.
Depends on D1214