od: add an --endian option to control byte swapping

od: add an --endian option to control byte swapping

* src/od.c (main): Handle the new --endian option,
taking "little" and "big" as parameters.
(usage): Describe the new option.
(PRINT_FIELDS): Adjust to swap bytes if required.
* tests/misc/od-endian.sh: A new test to verify
the byte swapping operations for hex (ints) and floats
for all sizes between 1 and 16 inclusive.
* test/local.mk: Reference the new test.
* doc/coreutils.texi (od invocation): Describe the new option.
* NEWS: Mention the new feature.