iterable_sections: move to specific header

Until now iterable sections APIs have been part of the toolchain
(common) headers. They are not strictly related to a toolchain, they
just rely on linker providing support for sections. Most files relied on
indirect includes to access the API, now, it is included as needed.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>