arch: arm: userspace: minor refactor in z_arch_is_user_context

Refactor z_arch_is_user_context() for ARM, so it uses
the CMSIS CONTROL_nPRIV_Msk instead of hard-coded 0x1.
Fixing also some typos in include/arch/arm/syscall.h.

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