lib: cpp: Relocate subsys/cpp to lib/cpp

This commit moves the files under `subsys/cpp` directory to the
`lib/cpp` directory because the C++ ABI runtime library and the
standard C++ library components are not a "subsystem" (aka. API) in
conventional sense and is better described as a "library."

Classifying the C++ ABI runtime library and the standard C++ library as
"libraries" instead of "subsystems" also better aligns with how the
existing C standard library implementation (`lib/libc`) is handled.

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