arch: arm: thread built-in stack guard implementation

This commit activates the built-in stack guard on the main_thread
before jumping to it upon system initialization. Stack guard is
activated if BUILTIN_STACK_GUARD is enabled by the user. The
commit also activates built-in thread stack guards at every
context switch, if BUILTIN_STACK_GUARD enabled by the user.

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