Add arm64 to libpng build configuration.

In libpng build option, neon files were missing for arm64,
add "arm64" option to libpng build configuration.

BUG=angleproject:4026

Change-Id: I38280c8089be93de224959583091f3a151f94ab0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1940017
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

third_party/libpng/BUILD.gn

@@ -24,7 +24,7 @@ static_library("libpng") {
     "src/pngwtran.c",
     "src/pngwutil.c",
   ]
-  if (target_cpu == "arm") {
+  if (target_cpu == "arm" || target_cpu == "arm64") {
     sources += [
       "src/arm/arm_init.c",
       "src/arm/filter_neon.S",