Default to WITHOUT_GDB (GDB 6.1.1) for FreeBSD 13

As discussed on -current, -stable, -toolchain, and with jhb@ and imp@,
disable the obsolete in-tree GDB 6.1.1 by default. This was kept only
to provide kgdb for the crashinfo tool, but is long-obsolete, does not
support all architectures that FreeBSD does, and held back other work
(such as forcing the use of DWARF2 for kernel debug).

Crashinfo will use kgdb from the gdb package or devel/gdb port, and will
privde a message referencing those if no kgdb is found.

Relnotes: Yes
Sponsored by: The FreeBSD Foundation