[zlib] Fix ARM32 GCC build.

It was attempting to use ARMv8 compiler flags.

Bug: https://github.com/dart-lang/sdk/issues/52080
Change-Id: I05f7d9fed42e9fbcabaff4f79bc604aad032e833
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5529831
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Cr-Commit-Position: refs/heads/main@{#1304129}
NOKEYCHECK=True
GitOrigin-RevId: 20908d3a960ba99246b09946895e771c8032cb7b