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)
MFH:		2023Q3