Enable NEON on all Flutter-supported ARM platforms

Since the conditional includes all Flutter target platforms, we
eliminate the target platform condition and include NEON for all ARM
platforms.

This is cleanup from 93ab170.

Bug: https://github.com/flutter/flutter/issues/62597
Change-Id: I921356518357e7731e45eeae5692932d63c6efa6
Reviewed-on: https://flutter-review.googlesource.com/c/third_party/libpng/+/34503
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Chris Bracken <cbracken@google.com>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index eb04ba2..3448491 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,8 +30,7 @@
     "pngwutil.c",
   ]
 
-  if ((is_android || is_linux || is_fuchsia || is_ios || is_mac || is_win) &&
-       (current_cpu == "arm" || current_cpu == "arm64")) {
+  if (current_cpu == "arm" || current_cpu == "arm64") {
     sources += [
       "arm/arm_init.c",
       "arm/filter_neon.S",