Add RISC-V support.

Add RISC-V support.

This supports both 0.11 and 0.13 versions of the debug spec.

Support for `-rtos riscv` will come in a separate commit since it was
easy to separate out, and is likely to be more controversial.

Flash support for the SiFive boards will also come in a later commit.

Change-Id: I1d38fe669c2041b4e21a5c54a091594aac3e2190
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/4578
Tested-by: jenkins
Reviewed-by: Liviu Ionescu <ilg@livius.net>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>