cmake: Extend cc-option support to C++

The target_cc_option[_fallback]() CMake extentions are now C++
aware. This means that they will now test options with both C and C++
compiler and include flags appropriately.

This fixes a warning that was issued when -Wno-pointer-sign was used
 with .cpp files.

cc1plus: warning: command line option ‘-Wno-pointer-sign’ is valid for
C/ObjC but not for C++

NB: This patch is designed to only affect CONFIG_CPLUSPLUS builds in
case there are any adverse affects with using generator expressions.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>