Refactor macros in pp_pack.c, removing support for mixed-endian byte orders.

Refactor macros in pp_pack.c, removing support for mixed-endian byte orders.

The byte-order handling can be simplified considerably if we don't have to
support mixed-endian systems. It's not clear whether Perl 5.000 even
compiled on PDP-11s, let alone more recent versions.

Support probably can be added back at the end of the refactoring, if someone
has time and a PDP-11 to test on, but for the intermediate stages it is a
lot easier not to need to think about such platforms.