cmake: Toolchain abstraction: Abstract compiler flag for add debug info

The macro, toolchain_cc_produce_debug_info, adds the compiler specific
flag for enabling the production of debugging information in the
toolchain native format.

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>