ART: Implement scalar loop unrolling.

Implement scalar loop unrolling for small loops
(on arm64) with known trip count to reduce loop check
and branch penalty and to provide more opportunities
for instruction scheduling.

Note: this functionality is turned off by default now.

Test: cloner_test.cc
Test: test-art-target, test-art-host

Change-Id: Ic27fd8fb0bc0d7b69251252da37b8b510bc30acc