Add fast path to arm64 READ_BARRIER macro

EAAC benchmark time from 978.7857143ms to 969.5714286ms on N9 based
on 42 samples. Reduces artReadBarrierSlow calls from 9M to 1M.

Not a huge improvement since we were already checking the lock word in
ReadBarrier::Barrier.

Test: N9 boots, test-art-host, EEAC runs. (All with CC enabled).

Bug: 30162165
Bug: 12687968

Change-Id: Ifb97b52ea84e21c7df83addfb91c5f05f41db32d