RISC-V GDB Port v3

[Palmer Dabbelt <palmer@dabbelt.com>]

Sorry it took me so long to get around to all the feedback, I was trying to
finish up grad school so I could start working full time on RISC-V software and
the GDB port fell through the cracks.  This is now part of my job, so hopefully
I'll be able to be better about this in the future.

I've attempted to fix all the outstanding comments with the v2 patch set, but
since it's been a while I'm not 100% sure I got everything.  Here's the
differences I remember since v2:

 * Changes the frame walking code to use riscv-opc.h instead of hard-coded
   constants.

 * Don't call deprecated functions when printing FP registers.

 * Use the sw_breakpoint_from_kind interface.

 * Add a RISC-V XML target description file, but I think it doesn't do anything
   yet.

I think the only thing I know of that's bad is the target XML description file,
which I don't think works yet.  That said, I want to send this patch set out
just so we can all get on the same page.

If I understand correctly, GDB-8 is set to be branched on the 15th so if I can
fix up our port quickly we can still get it in?

Sorry this took so long!

[PATCH 1/2] RISC-V GDB Port
[PATCH 2/2] RISC-V sim Port