[zlib_bench] Minor code maintenance and comment cleanup

Recent gcc require libs after .cc files: update the comment showing how
to compile outside of the chromium build system to match.

Minor usage_exit change: put the program options in a variable, outside
the function, to shorten the code and avoid an unfortunate line break.

Bug: 798943
Change-Id: I27e5e891d5dcc6c1f5ff872a517436f0c8853ef3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2870408


Auto-Submit: Noel Gordon <noel@chromium.org>
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#878743}
NOKEYCHECK=True
GitOrigin-RevId: 4895f17460d1a0140837f953086e69b816f6151b