cortex_m: fix bug in poll() machine state (external resume awareness)

cortex_m: fix bug in poll() machine state (external resume awareness)

This patch covers the fact that cortex_m could be resumed externally by
Cross Trigger Interface or by direct write to DHSCR ...

To reproduce:
 - halt the target
 - then run the core through DHCSR (mww 0xe000edf0 0xa05f0001)
 => this resumes the core, but target state in OpenOCD remains HALTED.

Change-Id: Ifa1ae18645bfeb863acc78a039bbf04873fd78fe
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/4817
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>