cmake: Toolchain abstraction: Introduce toolchain_cc_warning_extended

This is placeholder for extended warning flags, likely to change between
toolchains.

The intent here is to abstract Zephyr's dependence on toolchains,
thus allowing for easier porting to other, perhaps commercial,
toolchains and/or usecases.

No functional change expected.

Signed-off-by: Danny Oerndrup <daor@demant.com>
4 files changed