tests: iterable_sections: Add missing zephyr_iterable_section

As the test is for iterable sections, we need to utilize
zephyr_iterable_section for targets that need linker script
generation like arm-clang.

So add zephyr_iterable_section() for the RAM & ROM sections
that the testcases utilizes.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>