cpp: Fix toolchain C++ standard library linkage

The build system currently links the toolchain-provided C++ standard
library even when the C++ support (`CONFIG_CPP`) is not enabled.

This commit updates the build system to link the toolchain-provided C++
standard library when the C++ support is enabled and a C++ library
implementation other than the Zephyr minimal C++ library is selected.

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