Update crashinfo to work with newer gdb from ports.

[jhb]

Summary

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

Note that after this I think we can disable gdb by default on at least
amd64. In theory the 'gdbcommand' function is easily adaptable to lldb
if it supports 'printf' as a command. Still need a kernel target to
do the kernel backtrace though.

Test Plan

• Run modified crashinfo in a VM with the ports gdb installed and compare output with the stock crashinfo.
• Run modified crashinfo in a VM without ports gdb installed and comprae output with the stock crashinfo.