Add get_thread_reg() and set_reg() for hwthread

Add get_thread_reg() and set_reg() for hwthread

get_thread_reg() allows gdb to request the register value of a specific
"thread."
set_reg() allows register writes without getting a giant list of
registers first.

Signed-off-by: Tim Newsome <tim@sifive.com>
Change-Id: I87faa1c8793916b9ee476dd696f0695a07ca2b41
Reviewed-on: http://openocd.zylin.com/5324
Tested-by: jenkins