And you can see how glibc implements this for arm here:

http://repo.or.cz/w/glibc.git/blob_plain/a64e578b6fdb18c530d142a68a7e57eb04038b5d:/sysdeps/arm/bits/endian.h

which is why for my code I tested __ARMEB__, which is set by the compiler and not dependent on the libc implementation.