[krbdev.mit.edu #2551] Use __BIG_ENDIAN__ and __LITTLE_ENDIAN__ if available to detect endianness

[Ken Raeburn via RT]

The patch I just checked in for the db2 code doesn't check the
__FOO_ENDIAN__ macros, because we don't seem to need them so far.  It
would be reasonable to update both db2 and AES to check them, but I
don't see any hurry.  One minor benefit, if they're available, we could
do the test without including any additional system headers.