TurboFan currently calls into runtime for these two instructions, but
there is a better 4-instruction lowering that Liftoff already uses. Move
this into macro-assembler so we can share this across both compilers. We
name this PopcntHelper because there isn't a Cnt on ARM64 that works on
Word32/Word64.

Bug: v8:12071
Change-Id: I182bf466b76cbad985d8c5b8ddae0f4352f71cd2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3087812Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76236}