Port 5cc61896

Minor fix to the original CL's port of ppc/s390

Original Commit Message:

    This moves most of the logic contained in RegExpExecStub to CSA.  Benefits are
    mostly easier readability and hackability, and removal of a large chunk of
    platform-specific assembly.

    Exit frame construction and the final call remain in RegExpExecStub.

R=jgruber@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=v8:5339,v8:592
LOG=N

Review-Url: https://codereview.chromium.org/2757673004
Cr-Commit-Position: refs/heads/master@{#43877}