Remove pushes from memsets (krait/cortex-a9).

On the path that only uses r0 in both the krait and cortex-a9
memset, remove the push and use r3 instead.

In addition, for cortex-a9, remove the artificial function since
it's not needed since dwarf unwinding is now supported on arm.

Change-Id: Ia4ed1cc435b03627a7193215e76c8ea3335f949a