This is a good point, and I'd like to fix it. When I look for endianness macros, it doesn't seem like there's a common standard. Is there an approach you'd recommend?
It seems that you're right - ISO C doesn't have standard macros for this. That said, since this code is ARM-specific, and you are already leveraging the ACLE header <arm_neon.h>, I think that a negative test for __ARM_BIG_ENDIAN is appropriate.