add structure for normal SKX opts

My experience porting the old opts over to the current SKX opts setup
was so bad that I don't want to try any more, and think it's probably
safer to port the SKX code to the old setup.

Need to hook up dependents (Chromium and Google3 I think) before we can
move the actual SXK opts code over.

Change-Id: Ibb8bc4a083cb104cd39f27cbfbc16e9eedd9bd46
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/294495
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index b61f3e5..3c0405d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -218,6 +218,19 @@
   }
 }
 
+opts("skx") {
+  enabled = is_x86
+  sources = skia_opts.skx_sources
+  if (is_win) {
+    cflags = [ "/arch:AVX512" ]
+  } else {
+    cflags = [ "-march=skylake-avx512" ]
+    if (is_mac && is_debug) {
+      cflags += [ "-fno-stack-check" ]  # Work around skia:9709
+    }
+  }
+}
+
 # Any feature of Skia that requires third-party code should be optional and use this template.
 template("optional") {
   visibility = [ ":*" ]