This reverts commit 6afe7d18.

Reason for revert: Failures (-2e+66 vs. -2e+66): https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Linux%20-%20debug/22148

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}

TBR=titzer@chromium.org,clemensh@chromium.org

Change-Id: I4377c13346b42b65e8db04cbd15fc2f906113f65
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6600
Reviewed-on: https://chromium-review.googlesource.com/1219446Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55783}