ART: Fast copy stack mask

StackMap::SetStackMask will currently copy a BitVector into a Memory-
Region bit by bit. This patch adds a new function for copying the data
with memcpy.

This is resubmission of CL I28d45a590b35a4a854cca2f57db864cf8a081487
but with a fix for a broken test which it revealed.

Change-Id: Ib65aa614d3ab7b5c99c6719fdc8e436466a4213d