This is almost exclusively work from the VRULL team.

As we've discussed in the Tuesday meeting in the past, we'd like to have 
a knob in the tuning structure to indicate that overlapped stores during 
move_by_pieces expansion of memcpy & friends are acceptable.

This patch adds the that capability in our tuning structure.  It's off 
for all the uarchs upstream, but we have been using it inside Ventana 
for our uarch with success.  So technically it's NFC upstream, but puts 
in the infrastructure multiple organizations likely need.


Built and tested rv64gc.  Pushing to the trunk shortly.
jeff