devel/gdb: kgdb: fix aarch64 trapframe handling

regcache_map_entry_size() doesn't do the magic to convert size == 0 to
register_size, so we must supply sizes for all registers if we're going
to use it.

This fixes handling of the trapframe so that we can actually unwind
properly, where-as before we would end up with, e.g.,

#6 0xffff0000004b4860 in kdb_trap (type=60, code=0, tf=<optimized out>)

at /usr/src/sys/kern/subr_kdb.c:790

#7 <signal handler called>
#8 <signal handler called>
#9 <signal handler called>
#10 <signal handler called>
#11 <signal handler called>

Reported by: Mark Millard
Approved by: jhb (implicit, kgdb maintainer)

(cherry picked from commit a22f1db084fb50880a0df90f9a0f031a7fc34921)