Add a new KGDB option to the devel/gdb port. This adds a forward port
of the kernel-specific bits of kgdb to recent gdb. It only supports
amd64, i386, powerpc, powerpc64, and sparc64.

PR: 203299
Reviewed by: bdrewery
Glanced at by: emaste, jilles, luca.pizzamiglio@gmail.com (maintainer)
Differential Revision: https://reviews.freebsd.org/D3727