MFC r329989, r330044

r329989:
Add support for booting into kdb on arm platforms when the RB_KDB is set
(using "boot -d" at the loader propmt or setting boot_ddb in loader.conf).

Submitted by: Thomas Skibo <thomasskibo@yahoo.com>
Differential Revision: https://reviews.freebsd.org/D14428

r330044:
Add a hw.model sysctl oid for armv6/7 which reports the CPU model, similar
to what other arches (all except riscv and armv4/5) do.

Submitted by: Hyun Hwang <hyun@caffeinated.codes>
Differential Revision: https://reviews.freebsd.org/D14465