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

In this commit we implement the assembly functions in
swap_helper.S, namely
  - z_arm_pendsv()
  - z_arm_svc()
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>