doc coding_guidelines: Exclude host tooling from rule A.5

Rule A.5 is meant for code build with the embedded libCs and
which runs in embedded targets. Let's be more clear about this
and explicitly indicate that host tooling is not covered by it.
Otherwise, the current
"The "Zephyr codebase" in this context refers to all source
code files committed to the main Zephyr repository"
covers too much.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>