Fix the stack at the beginning of the Generic JNI trampoline.

Fix up a callee-save frame at the bottom of the stack while we
check for optimization annotations, thus allowing stack walking
until the completion of the JNI frame creation.

Test: art/test/testrunner/testrunner.py -t 656-annotation-lookup-generic-jni
Bug: 38454151
Bug: 34659969
Change-Id: I70dc3d40139198f29457dba5282b45a9d0d09e49
Merged-In: I70dc3d40139198f29457dba5282b45a9d0d09e49