Like the other Math builtins, Math.abs() is now a TurboFan builtin.
It uses RawMachineAssembler::Float64Abs().

R=bmeurer@chromium.org
BUG=v8:5163, v8:5086
LOG=N

Review-Url: https://codereview.chromium.org/2115493002
Cr-Commit-Position: refs/heads/master@{#37433}