arm: Do not allow PNG_ARM_NEON_CHECK_SUPPORTED on ARM64

ARM Neon is expected to be unconditionally available on ARM64.
Issue an #error if PNG_ARM_NEON_CHECK_SUPPORTED is defined on an
ARM64-based platform.

For the plain ARM platforms where ARM Neon checking is not yet
implemented, issue a slightly more descriptive #error, including
a suggested course of action.
1 file changed