Also unify the Pow implementation somewhat. There are still some
inconsistencies with the FPU version for x64/ia32, but that has
to be resolved separately.

R=ahaas@chromium.org, mvstanton@chromium.org
BUG=v8:5086

Review-Url: https://codereview.chromium.org/2333663002
Cr-Commit-Position: refs/heads/master@{#39368}