These files are only required if `PNG_ARM_NEON_IMPLEMENTATION == 1` (the default). Since we can't test it here, we just have to assume it's the case on all Arm devices. (But only Arm devices.)
As noted in pngpriv.h, the alternative (assembly code) doesn't currently
work on arm64.
There's no mention of iOS or tvOS, so maybe we could remove those?