cmake: Toolchain abstraction: Abstraction of implicit-int error flag

Introducing the macro toolchain_cc_warning_error_implicit_int which,
abstracts the implicit_int error flag thus leaving it to the toolchain
to decide whether this flag is needed or not.

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>