Fix alignment of Code::kConstantPoolOffset.

This is needed for 64bit alignment sensitive platforms (PowerPC)
Exposed bugs with new GC compare and swap changes updating the
field.

Example failing test:
out/ppc64.debug/cctest  test-decls/ExistsInPrototype

BUG=

Review URL: https://codereview.chromium.org/740443002

Cr-Commit-Position: refs/heads/master@{#25472}