Allow libpng to be compiled with `-mfpu=neon`

This change is otherwise innocuous because of the `PNG_ARM_NEON_OPT`
check in both new source files. So it will still compile fine
both for non-NEON ARM and for non-ARM architectures.
Merge pull request #1 from thotypous/master

Allow libpng to be compiled with `-mfpu=neon`