This adds support for f32.neg and f64.neg. Note that this cannot be
computed as "0 - src", as this would not turn 0 into -0. Instead, we
need to explicitly flip the sign bit.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I3cbcfa156c5d2a727e0e2da279369bf055f0d657
Reviewed-on: https://chromium-review.googlesource.com/937202Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51601}