lib: cpp: Rework C++ standard library configurations

This commit reworks the C++ standard library configurations such that:

* the separation between the Zephyr minimal C++ library and the fully
  featured C++ standard libraries, such as GNU libstdc++, is clear.
  This is done by deprecating the Kconfig `CONFIG_LIB_CPLUSPLUS`
  symbol, which implies that the minimal C++ library is selected when
  set to `n`, and introducing the `CONFIG_MINIMAL_LIBCPP` symbol.

* the type of the selected C++ standard library is clear. This is done
  by introducing a Kconfig choice, `LIBCPP_IMPLEMENTATION`, for the C++
  standard library type and providing the choice symbols for each
  library type supported, such as `CONFIG_MINIMAL_LIBCPP` and
  `CONFIG_GLIBCXX_LIBCPP`.

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