armv7m: always set xPSR.T=1 when starting an algorithm

armv7m: always set xPSR.T=1 when starting an algorithm

xPSR.T sets the processor to Thumb mode when set to 1. ARMv7-M only
supports execution of Thumb instructions, so it must always be set to 1.

If xPSR.T is set to 0 on armv7m, a usage fault is generated when a
instruction execution is attempted.

On armv7m, issuing a reset causes the vector table to be examined. PC
and xPSR.T  are loaded from the vector table at byte offset 4. xPSR.T is
taken from the least significant bit this value, PC from the remaining
bits.  This occurs even with `reset halt`, as the reset itself causes
this load to occur without the execution of any instructions.

As a result of this, following a reset with a "bad" value programmed in
the vector table, openocd would be unable to run algorithms on the
target, as running them would immediately result in a usage fault due to
xPSR.T being unset (0).

Allow algorithms to run regardless of the content of the vector table by
explicitly setting xPSR so that xPSR.T=1 prior to executing an
algorithm. One can think of this as openocd more closely emulating a
reset or branch instruction in executing it's algorithms.

Ticket: https://sourceforge.net/p/openocd/tickets/203/
Signed-off-by: Cody P Schafer <openocd@codyps.com>
Change-Id: I4dc3427ab195d06c3fd780ea768027fefccc4c28
Reviewed-on: http://openocd.zylin.com/4658
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>