target/cortex_m: fix target_to_cm() helper

The third parameter of container_of() should point to the same member
as target->arch_info points to, struct arm.

It worked just because struct arm is the first member in
struct armv7m_common.
If you move arm member from the first place, OpenOCD fails heavily.

Change-Id: I0c0a5221490945563e17a0a34d99a603f1d6c2ff
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6749


Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>