mac/arm64: Make zlib actually link.

This is a follow-up to https://chromium-review.googlesource.com/c/chromium/src/+/2271018
which made the initialization at arm_cpu_enable_crc32 build-time constant for mac/arm64.
However, since there are a few callers of cpu_check_features() in cross-platform
zlib code, we still need to give cpu_check_features() an (empty) implementation
on mac/arm. (...or add preprocessor checks for mac/arm at each calling site,
which seems worse.)

Bug: 1099892
Change-Id: I65785c2e815d80d7e50f25280963ed5aba66304c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2284145


Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#785570}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 64cf0064494d35e48dcb9cf253e18b270fa58c57