rtems gdb stub with virtex issue

[timcussins at eml.cc (Tim Cussins)]

Hi everyone,

I've sussed out the issue with rtems-stub on virtex - it requires
changes on two fronts to get it going.

AFAICT, the powerpc standard doesn't actually specify a standard for
hardware debugging support, so the ppc405 is a slightly different beast
to the mpc7455 (used in the mvme5500 bsp) when it comes to debug
flags/switches/modes etc.

This means an addition to rtems (in new-exceptions) and changes in the
gdb stub (in the arch-dependent ppc code).

I'm working with the 4.8 branch atm, and have developed a patch to add
support for a further ppc405 exception: the 'debug' exception. Yep, I
was as surprised as you are. ;)

The arch-dependent part of the stub needs a some modifications to
support the new 'debug' exception.

Stepping/breakpoints are mostly all go, but there's still some oddness
when single-stepping that I need to sort out. I'll keep you all posted.

Later,
Tim