rtos: support gdb_get_register_packet

rtos: support gdb_get_register_packet

This patch adds support for p packet responses by targets configured
with RTOS support. This change required moving to a rtos_reg struct,
which is similar to struct reg used by targets, which resulted in
needing to update each stacking with register numbers. This patch also
allows targets with non-linear register numbers to function with RTOSes
as well.

Change-Id: I5b189d74110d6b6f2fa851a67ab0762ae6b1832f
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/4121
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>