Fix building libpng on arm CPUs.
Fix building libpng on arm CPUs.

Some arm-specific files were not part of the build.

BUG=angleproject:4026

Change-Id: Ie3534c66ebce53348a00f74b95b7f6e3859ee30f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1866079
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle