arch: arm: cortex-m: extract common code block into a static function

We extract the common code for both multithreading and
non-multithreading cases into a common static function
which will get called in Cortex-M archictecture initialization.
This commit does not introduce behavioral changes.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>