[Bug 292494] powerpc64: booting qemu + mac99 results in crash (kernel DSI write trap) after panic: witness_save: witness_cold

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=292494

            Bug ID: 292494
           Summary: powerpc64: booting qemu + mac99 results in crash
                    (kernel DSI write trap) after panic: witness_save:
                    witness_cold
           Product: Base System
           Version: 16.0-CURRENT
          Hardware: powerpc
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: powerpc
          Assignee: [email protected]
          Reporter: [email protected]

Created attachment 267180
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=267180&action=edit
crash in early boot

I'm not sure where this bug is yet, but it's super easy to reproduce in QEMU.

qemu config:

```
qemu-system-ppc64 \
        -machine mac99,via=pmu \
        -s -S \
        -cpu 970fx \
        -smp 1 \
        -drive file=wd0.img,format=raw,media=disk,snapshot=on \
        -drive
file=../iso/FreeBSD-16.0-CURRENT-powerpc-powerpc64-20260113-2b60e628d3b1-283055-bootonly.iso,format=raw,media=cdrom,readonly=on,index=2
```

The snapshot from 20260113 has the image generation fix so it does boot, but it
ends up crashing super early in boot and crash loops.

I ended up attaching gdb to it (the -s -S in the above config) and through
trial and error figured out that the putc code is at 0xc0000000006d4550, so put
a breakpoint there and just watched it print the panic character by character.

To attach kgdb:

```
$ kgdb
(kgdb) set endian big
(kgdb) set architecture powerpc:common64
(kgdb) target remote localhost:1234
(kgdb) breakpoint *0xc0000000006d4550
(kgdb) cont
```

and let it boot and run.

I've attached a screenshot showing the crash.

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