target/armv7m,cortex_m: introduce checked arch_info cast routines

target/armv7m,cortex_m: introduce checked arch_info cast routines

target_to_armv7m() and target_to_cm() do not match the magic number
so they are not suitable for use outside of target driver code.

Add checked versions of pointer getters. Match the magic number
to ensure the returned value points to struct of the correct type.

Change-Id: If90ef7e969ef04f0f2103e0da29dcbe8e1ac1c0d
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6750
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>