arm64, riscv: handle RB_KDB

This allows entering the debugger at the earliest possible time, if
the '-d' argument is passed to the kernel.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D34120

(cherry picked from commit 4e1bc961bb4467f074e48aa6e710ccd1aa7f2888)