ARM64: don't try to use temp register when it ...

... might not be available.

Temp register might not be needed if offset in Unsafe.get is
constant.

Bug: n/a
Test: ART_USE_READ_BARRIER=true ./art/test/testrunner/testrunner.py -b --target --64
Test: ART_USE_READ_BARRIER=true ./art/test/testrunner/testrunner.py -b --host --64
Test: ART_USE_READ_BARRIER=true ./art/test/testrunner/testrunner.py -b --host --32
Test: ./art/test/testrunner/testrunner.py -b --target --64
Test: ./art/test/testrunner/testrunner.py -b --host --64
Test: ./art/test/testrunner/testrunner.py -b --host --32
Test: ./art/tools/run-libcore-tests.sh --mode=host --variant=X64
Test: ./art/tools/run-libcore-tests.sh --mode=host --variant=X32
Flag: EXEMPT BUGFIX

Change-Id: If52f374901d9c54f6422d93acb1dbed0b83dfa43