kconfig: cpp: Have LIB_CPLUSPLUS depend on ! MINIMAL_LIBC

The C++ STD library is not compatible with the minimal C library. To
ensure that users do not accidentally enable LIB_CPLUSPLUS while also
enabling a minimal libc library we add a dependency in Kconfig.

Also, use depends instead of select between EXCEPTIONS, RTTI, and
LIB_CPLUSPLUS as use of select is discouraged in this situation.

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