arm: add options GDB to std.armv6 and std.armv7

There are now options for specifying the debug port via tunable
(hw.fdt.dbgport) and device tree (/chosen/freebsd-dbgpath), so this can
be usefully included in GENERIC.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29152