target/cortex_m: Implement maskisr steponly option

`maskisr steponly` disables interrupts during single-stepping but
enables them during normal execution. This can be used as a partial
workaround for 702596 erratum in Cortex-M7 r0p1. See "Cortex-M7 (AT610)
and Cortex-M7 with FPU (AT611) Software Developer Errata Notice" from
ARM for further details.

Change-Id: I797a14e4d43f6dcb3706528ee4ab452846ebf133
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4673


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>