compat/bswap.h: detect endianness on more platforms that don't use BYTE_ORDER

Signed-off-by: Charles Bailey <cbailey32@bloomberg.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
compat/bswap.h: restore preference __BIG_ENDIAN over BIG_ENDIAN

The previous commit swaps the order we check the macros defined by
the compiler and the system headers from the original.  Since the
order of check should not matter (i.e. it is insane to define both
__BIG_ENDIAN and friends and BIG_ENDIAN and friends and in a
conflicting way), it is the most conservative thing to do not to
change it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'cb/byte-order' into jch

* cb/byte-order:
  compat/bswap.h: restore preference __BIG_ENDIAN over BIG_ENDIAN
  compat/bswap.h: detect endianness on more platforms that don't use BYTE_ORDER