Update crashinfo to work with newer gdb from ports.

If gdb from ports is installed, use it instead of the base system gdb
to extract variables from a kernel. Note that base gdb and ports gdb
do not support the same options for invoking a single command in batch
mode, so a wrapper shell function is used. In addition, prefer kgdb
from ports when generating a backtrace if present.

PR: 193335
Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D7218