This is a reland of 5ae8420c

Original change's description:
> [Liftoff] Implement float rounding
> 
> 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}

Bug: v8:6600
Change-Id: I9e69309b71041e411bda9ce1375d2451197a111b
Reviewed-on: https://chromium-review.googlesource.com/997393
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52382}