target/cortex_m: restore C_MASKINTS after reset

target/cortex_m: restore C_MASKINTS after reset

The cortex_m maskisr user-facing setting is not changed across a target
reset. However, the in-core C_MASKINTS bit was always cleared as part of
reset processing, meaning that a cortex_m maskisr on setting would not
be respected after a reset. Set C_MASKINTS based on the user-facing
setting value rather than always clearing it after reset.

Change-Id: I5aa5b9dfde04a0fb9c6816fa55b5ef1faf39f8de
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4605
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>