lib: cpp: Relocate C++ minimal library components to lib/cpp/minimal

This commit relocates the "C++ minimal library" components, that
implement a very limited subset of the standard C++ library, to a
dedicated directory, `lib/cpp/minimal`, in order to provide a clear
separation among the different C++ library components.

After this refactoring, the top `lib/cpp` directory should only contain
the sub-directories for each C++ library (i.e. `abi`, `minimal`). In
the future, a C++ library-specific shim layer implementation may be
added as sub-directories under `lib/cpp` as well (e.g. `libstdc++`
sub-directory containing the shim layer implementation for the GCC
libstdc++ library) -- this is similar to how the libc directories are
structured under `lib/libc`.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>