Fix a couple of issues in kgdb.

[jhb]

Summary

• Properly unwind across in-kernel exceptions on arm.

• Enumerate processes via the pid hash table for kernels without the zombproc list.

PR: 251463 (1)

Test Plan

• I've tested the zombproc fix on a recent panic on head during exit1() where kgdb didn't see the panicking thread without the fix.

• The OP for the PR has not tested the arm patch, but I beleive it to be correct.

[pizzamig]

It looks good to me.
I tested the build on the available platform and I don't have a way to test on arm.
However, it's good to be committed