This is a reland of 6afe7d18.
The reason for the revert is fixed in https://crrev.com/c/1219633.

Original change's description:
> [Liftoff] Implement f32.copysign and f64.copysign
>
> These are two of the few missing instructions. This CL implements them
> for ia32 and x64, and bails out on other platforms.
> On x64, we are using the BTR instruction since we cannot have 64-bit
> immediates.
>
> Drive-by: Fix naming of existing bt/bts instructions on x64.
>
> R=titzer@chromium.org
>
> Bug: v8:6600
> Change-Id: Ib8532ca811160cd61f4ba7c06b04ce093861c872
> Reviewed-on: https://chromium-review.googlesource.com/1174383
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55780}

Bug: v8:6600
Change-Id: I4baeec6b02b17450988cfa7fedd5037f9cfe1638
Reviewed-on: https://chromium-review.googlesource.com/1219508Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55812}