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)

devel/gdb: kgdb: fixup aarch64 pcb/trapframe layout

base 1c1f31a5e5 and 2ecbbcc7cab altered the pcb and trapframe layouts in
osreldate 1400084. This version of the patch is effectively extracted
from work done by @jhb in CheriBSD's gdb fork, then ported forward to
work with the gdb 13.x program_space_data equivalent. With
this + D39951, I can again get a solid backtrace in recent main.

While we're here, remove some special handling for kthreads not yet run,
as the stack pointer is in-fact stored in pcb_sp in cpu_copy_thread and
cpu_fork.  This silences the following exception:

Python Exception <class 'gdb.error'>: Register 2 is not available

but there's not really any effect on the end result, since the thread
hasn't been scheduled yet.

Reviewed by:	jhb
Approved by:	jhb (maintainer)

(cherry picked from commit 5dd228e81c1bcb49e0a7c5eca3927df24a607a15)

devel/gdb: kgdb: Handle stoppcbs compat

Use __FreeBSD_version to decide whether to dereference the stoppcbs
symbol.  PORTREVISION will be bumped as part of the next commit, which
fixes another KBI breakage.

Reviewed by:	jhb, kevans
Approved by:	jhb (maintainer)

(cherry picked from commit 7e2852846872215c7d7ad36d9ea15d593bbf343e)