RISC-V: linux native port started

This is an FYI for anyone that might be interested.  I've started work
on a RISC-V linux native gdb port.  I just have basic functionality
working.  I can read/write integer registers.  I can set breakpoints,
run programs, and continue after the breakpoint.  There is probably
not much else working at the moment.  I expect that I will continue
working on this as a side project, as binutils and gcc are my primary
focus, and I will try to submit a patch later when it is cleaner and
working better.

There is one linux kernel patch I needed to get this far, for RISC-V
PTRACE_SETREGSET support, and has already been submitted to the linux
kernel maintainers.  I expect more linux kernel patches will be
required later, for instance to read/write FP registers.

Jim