cortex_m: use cortex_m_write_debug_halt_mask() in cortex_m_single_step_core()

cortex_m: use cortex_m_write_debug_halt_mask() in cortex_m_single_step_core()

cortex_m_single_step_core() used mem_ap_write_atomic_u32() to manipulate
dhcsr bits unlike the rest of code, where a specialized function
cortex_m_write_debug_halt_mask() takes place.

Unify setting of dhcsr bits and use cortex_m_write_debug_halt_mask() here as well.

Extracted from [1].

[1] Antonio Borneo: 6207: cortex_m: rework handling of dcb_dhcsr

Link: https://review.openocd.org/c/openocd/+/6207
Change-Id: I9ef05ce88a9dce42e1d3d5404a4fe87ec86b5fe8
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6676
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>