tests: arch: arm_irq_vector_table: enable test for ARMv6-M CPUs

This commit enables running the test in CI for Cortex-M Baseline
architecture. As STIR register is not present in Baseline cores,
we need a minor modification in the test code so it uses NVIC
intrisics instead of setting the STIR for ARMv6-M.

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