Replace -march= with -mfeature toggles.

Some build configurations (e.g. ChromeOS) explicitly add an
-march=x86-64 to the build flags, which overrides our
-march=haswell/skylake setting. However, it is still safe to enable
feature flags like -mavx2; -march=x86-64 will not un-set them.

This will hopefully help unblock the skcms -> Skia roll.

Bug: b/310927123
Change-Id: I3d6eb5b7ea2f082a0cfa4923ddaa571eec4e7cdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5046492
Commit-Queue: Kevin Lubick <kjlubick@chromium.org>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Kevin Lubick <kjlubick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1226996}