[PPC] Fix register context fetch from pcb

[luporl]

Summary

This change fixes 2 issues in ppcfbsd_supply_pcb():

• Wrong indexing of pcb.pcb_context[]
• Initial PC register was not being supplied

With this change, backtrace command works on kgdb, with a live kernel or a crashdump.

Symbol resolution, however, still won't work.
This will be addressed in D21946, from which this change was a part of, but is now in its own separate revision, as it is not related to the symbol resolution issue.

[jhb]

The kgdb bits actually live in github at https://github.com/bsdjhb/gdb/tree/kgdb-8.3 If it's not too hard to make a pull request against that, that would be nicer.

The other thing to consider after that is changing it to be cross-debug friendly. I can help with that. It means not using struct pcb but using a table describing the offsets of the registers from the pcb address.

[luporl]

In D22761#498280, @jhb wrote:

The kgdb bits actually live in github at https://github.com/bsdjhb/gdb/tree/kgdb-8.3 If it's not too hard to make a pull request against that, that would be nicer.

The other thing to consider after that is changing it to be cross-debug friendly. I can help with that. It means not using struct pcb but using a table describing the offsets of the registers from the pcb address.

Ok, I made the pull request there, with these changes.

Can you give me some guidelines of the appropriate way to create a table with struct pcb offsets?

[luporl]

Replaced by D23844.