ART: Fix inline assembly constraints in stub_test

GCC 4.9 is the first compiler actually using the (incorrect)
absence of the memory clobber constraint to elide some stores
and reloads to the thread, leaving garbage in there on exit
of the invoke method when the called stub will reach C code
with FinishCalleeSaveFrame.

Change-Id: I3afe9926bf537e6e92e3ad44eb6541776ac0e75f