gdb_server, rtos: Fine-grained RTOS register access

gdb_server, rtos: Fine-grained RTOS register access

1. Add get_thread_reg() to rtos. It's used in rtos_get_gdb_reg() to read
the value of a single register, instead of reading all register values
by calling get_thread_reg_list().
2. Add set_reg() to rtos. gdb_server uses this to change a single
register value for a specific thread.
3. Add target_get_gdb_reg_list_noread() so it's possible for gdb to get
a list of registers without attempting to read their contents.

The clang static checker doesn't find any new problems with this change.

Change-Id: I77f792d1238cb015b91527ca8cb99593ccc8870e
Signed-off-by: Tim Newsome <tim@sifive.com>
Reviewed-on: http://openocd.zylin.com/5114
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>