ARM64: remove stack pushes without frame in RegExpExecStub.

RegExpExecStub pushes callee-saved registers without setting up a frame. This confuses the stack iterator.

Other architectures do not save these registers.

BUG=chromium:487981
LOG=NO
TEST=mjsunit/regress/regress-487981

Review URL: https://codereview.chromium.org/1183593005

Cr-Commit-Position: refs/heads/master@{#29120}