PPC: perf enhancement: Use larger heap page size on PPC.

PPC: perf enhancement: Use larger heap page size on PPC.

Revisit of https://codereview.chromium.org/910333004.

Use 4MB heap page size over the default of 1MB.

This change provides an improvement of 1.86% on the composite octane
benchmark score on PPC. This is 0.56% more than if --min_semi_space_size=4
was used to specify a 4MB heap page size.

Additionally, two more tests required modification to account for
configurable heap page size.

R=svenpanne@chromium.org, hpayer@chromium.org, danno@chromium.org, mbrandy@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1221433022

Cr-Commit-Position: refs/heads/master@{#29775}