Minor improvements in JNI assemblers.

Address issues identified in
    https://android-review.googlesource.com/1246286
and clean up a few other things.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Test: aosp_taimen-userdebug boots.
Test: run-gtests.sh
Test: testrunner.py --target --optimizing
Bug: 12189621
Change-Id: I9ee1a9c113ef756d7aa3bd4d3f17ef1aaa4306f5
9 files changed