cksum: fix --raw on 64 bit big endian systems

cksum: fix --raw on 64 bit big endian systems

* src/sum.c (output_bsd): On sparc64 for example,
a crc of 0 was output due to casting an int variable
to uint16_t and thus operating on the wrong end of the variable.
Instead use explicit assignment to the narrower type
to ensure we get the appropriate data.
(output_sysv): Likewise.
Reported by Bruno Haible.