crc32: use __BYTE_ORDER macro for endian detection.

Since crc32.c contains a nifty test program that can be executed in user
space, make sure endian detection works reliably in user space too.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>