arch: arm: userspace: adapt assembly code for Cortex-M Baseline

In this commit we implement the assembly functions in userspace.S
- z_arm_userspace_enter()
- z_arm_do_syscall()
- z_arch_user_string_nlen()
for ARMv6-M and ARMv8-M Baseline architecture. We "inline" the
implementation for Baseline, along with the Mainline (ARMv7-M)
implementation, i.e. we rework only what is required to build
for Baseline Cortex-M.

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