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}