Cortex-M: fix stale DHCSR cache values

Cortex-M: fix stale DHCSR cache values

In cortex_m_assert_reset, in two locations, DHCSR is written directly
using mem_ap_write_u32. This means that the cached version,
target_to_cm(target)->dcb_dhcsr, is not updated when these writes are
performed, so subsequent writes to DHCSR that use
cortex_m_write_debug_halt_mask will change those bits back to their old
values which, unless modified in that particular invocation, come from
the cache. This causes an actual, observable bug on an STM32F7 in which
running “reset run” immediately after “program” can in some cases result
in execution proceeding with C_MASKINTS set (it is cleared on line 1021
but is then set immediately afterward in cortex_m_clear_halt), causing
failure of the application. Replace these mem_ap_write_u32 calls with
cortex_m_write_debug_halt_mask calls to do the same jobs.

Change-Id: Id35ca7f6057c2df2ba9cd67c53a73b50816d0b71
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/4239
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>