This fixes the signature of "Math.abs" from "(signed) -> signed" to
"(signed) -> unsigned" and hence fixes cases where the absolute value
would overflow the range of signed 32-bit values. This is in sync with
spec erratas (and ECMAScript semantics).

Note that this also switches the underlying implementation of the above
absolute value function to a branch-free version.

R=clemensh@chromium.org
TEST=mjsunit/regress/regress-6838-3
BUG=v8:6838

Change-Id: Ib13b7ecd336ae386cbde7c574e727bf52f841e00
Reviewed-on: https://chromium-review.googlesource.com/684181
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48169}