Prefer Pow() as it works around certain cases that are different in AIX's
std::pow().

TEST=mjsunit/harmony/exponentiation-operator
R=caitpotter88@gmail.com, littledan@chromium.org, adamk@chromium.org, rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1916043002

Cr-Commit-Position: refs/heads/master@{#35775}