od: omit some duplicate code

od: omit some duplicate code

On x86-64 (for example) print_long, print_long_long, and
print_intmax all behave identically, so give GCC enough info so
that it generates code for just one of these functions.
* src/od.c (enum size_spec): Arrange for enum values to
be the same if they represent types that behave the same.
(width_bytes, ISPEC_TO_FORMAT, decode_one_format):
Match the enum size_spec changes.