This adds support for the four rounding operations on f32 and f64:
f32.ceil, f32.floor, f32.trunc, f32.nearest_int, f64.ceil, f64.floor,
f64.trunc, and f64.nearest_int.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I16ea91e26c3233af7a33bbb794cd4182f9d7cdb0
Reviewed-on: https://chromium-review.googlesource.com/995894
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52377}