This adds support for i32.popcnt. If no hardware instruction for popcnt
is available, call out to C.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I9ae9e1d1e1392168d19c0eedcdd33eeea609a54f
Reviewed-on: https://chromium-review.googlesource.com/860658
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50528}