PPC: Debugger: use debug break slots to break at function exit.

PPC: Debugger: use debug break slots to break at function exit.

Port fc9c5275c3a747caca709b7d5745579f70e61301

Original commit message:
    By not having to patch the return sequence (we patch the debug
    break slot right before it), we don't overwrite it and therefore
    don't have to keep the original copy of the code around.

R=yangguo@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1238503003

Cr-Commit-Position: refs/heads/master@{#29703}