test_mem_safe: support ARM

Test works for ARM, but not testing sys_mem_safe_write_to_text_section()
since base ARM image are XIP images.

This test could be enhanced to  boot a secondary non-XIP image for ARM
to test writing to .text.

Change-Id: I56aef68afabe88588b61c273901491de0c24a39b
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>