arch: arm: cortex_m: pm_s2ram: Rework S2RAM mark functions

The S2RAM procedure requires marker checking after reset.
Such checking is performed on very early stage of the system initialization
and must ensure that the stack is not used due to the TLS pointer which is
not initialized yet.

Signed-off-by: Adam Kondraciuk <adam.kondraciuk@nordicsemi.no>