kdump: use VIS_NOLOCALE when printing output

Attempting to parse the output as UTF-8 exposed an out-of-bounds read
in vis(3) (see https://reviews.freebsd.org/D30771). However, I don't
think it makes much sense to decode this output at all. Additionally,
passing one byte at a time will almost certainly result in incorrect
output for multi-byte characters.

Reviewed By: brooks
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D30772