cortex_m: Fix single stepping will not return to debug mode sometimes

This occurs when stepping past a breakpoint on a even address with
maskisr option set to auto

With -d3 the following log message appears in this case:

"Debug : Interrupt handlers didn't complete within time,
 leaving target running"

Cause : Given a breakpoint is set on the lower half word and the PC is on
the upper half word. When another breakpoint is now set on the current PC
then resuming the core will not result in a break on the newly set
breakpoint. This has been observed on a STM32F1x, STM32F2x (CM3) but not
on a STM32F0x (CM0). It's not clear if this is a STM32F1/F2 only or a
general CM3 problem.

Change-Id: I384813f3bfdf935373b5e23cdb2d7f243c70cc00
Signed-off-by: Peter Horn <peter.horn@bluewin.ch>
Reviewed-on: http://openocd.zylin.com/864
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>