Introduce a new machine operator Float64Pow that for now is backed by
the existing MathPowStub to start the unification of Math.pow, and at
the same time address the main performance issue that TurboFan still has
with the imaging-darkroom benchmark in Kraken.

Also migrate the Math.pow builtin itself to a TurboFan builtin and
remove a few hundred lines of hand-written platform code for special
handling of the fullcodegen Math.pow version.

BUG=v8:3599,v8:5086,v8:5157

Review-Url: https://codereview.chromium.org/2103733003
Cr-Commit-Position: refs/heads/master@{#37323}