In pp_pack.c, refactor DO_BO_(UN)?PACK to use my_letohn etc

In pp_pack.c, refactor DO_BO_(UN)?PACK to use my_letohn etc

Previously they were using a my_letoh* function for the appropriate size.

This commit probably breaks fixed 16 and 32 bit templates on big endian
Crays where sizeof(short) and sizeof(int) are 8. A future commit will
restore support. (Although it's not clear whether such Crays can still
build blead cleanly, as we've not had any feedback since Boeing
decommissioned their Cray.)