This adds support for the f32.abs and f64.abs opcodes.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I05a16bb4301d492ba8d22a6326c7b2ce0f9f2faa
Reviewed-on: https://chromium-review.googlesource.com/968502Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52051}