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>