Enable NEON support on MacOS / ARM64

Required to support building for M1 Macs.

Bug: https://github.com/flutter/flutter/issues/69221

Patch authored by Matej Knopp <matej.knopp@gmail.com>

Change-Id: I78fd38d31c7b1a2ea16af651f6b950888511bd44
Reviewed-on: https://flutter-review.googlesource.com/c/third_party/libpng/+/12660
Reviewed-by: Kaushik Iska <kaushikiska@google.com>
Commit-Queue: Chris Bracken <cbracken@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 5cc2733..d56c37c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,7 +30,7 @@
     "pngwutil.c",
   ]
 
-  if ((is_android || is_linux || is_fuchsia || is_ios) &&
+  if ((is_android || is_linux || is_fuchsia || is_ios || is_mac) &&
        (current_cpu == "arm" || current_cpu == "arm64")) {
     sources += [
       "arm/arm_init.c",