PPC: Support fast-path allocation for subclass constructors with correctly initialized initial maps.

Port b9d25d86

R=verwaest@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com, dstence@us.ibm.com
BUG=v8:3330
LOG=n

Review URL: https://codereview.chromium.org/1434793002

Cr-Commit-Position: refs/heads/master@{#31926}