Revert "back to individual flags for :hsw"

This reverts commit 17c1cd4d55.

Reason for revert: good to try again.

Original change's description:
> back to individual flags for :hsw
>
> We still have some bots that use GCC 4.8 that don't understand
> -march=haswell.  I hope to fix that, then revert this.
>
> Change-Id: I8ba99d7170d00945e07da33b893a63981b64f5ca
> Reviewed-on: https://skia-review.googlesource.com/115761
> Reviewed-by: Mike Klein <mtklein@chromium.org>
> Commit-Queue: Mike Klein <mtklein@chromium.org>

Change-Id: Ib7b970a2987a69e4c12b2284e1f983f584bca076
Reviewed-on: https://skia-review.googlesource.com/116100
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>

BUILD.gn

@@ -316,11 +316,7 @@ opts("hsw") {
   if (is_win) {
     cflags = [ "/arch:AVX2" ]
   } else {
-    cflags = [
-      "-mavx2",
-      "-mf16c",
-      "-mfma",
-    ]
+    cflags = [ "-march=haswell" ]
   }
 
   # Oddly, clang-cl doesn't recognize this as a valid flag.