Adjust the sopt_val pointer on bigendian systems (e.g. MIPS64EB).

sooptcopyin() checks if size of data provided by user is <= than we can
accept, else it strips down the size. On bigendian platforms we have to
move pointer as well so we copy the actual data.

Reviewed by: gnn
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5
Differential Revision: https://reviews.freebsd.org/D7980