extend -O1 hack to avx2 code as well

Remember this bug where things would draw completely
right then wrong then right then wrong in vertical
stripes, that boiled down to an inappropriate call
to vzeroupper before some Mac-specific stack checks?

At the time we thought it was limited to only AVX
code, but of course it applies to AVX2 as well.

This removes pinstripes from many GMs when at -O0.

Bug: skia:9709
Change-Id: I51bc347529b4e3b6c00db37f9cfa18230a614345
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265900
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>

BUILD.gn

@@ -200,6 +200,9 @@ opts("hsw") {
     cflags = [ "/arch:AVX2" ]
   } else {
     cflags = [ "-march=haswell" ]
+    if (is_mac && is_debug) {
+      cflags += [ "-O1" ]  # Work around skia:9709
+    }
   }
 }