cmake: toolchain: Introduce TOOLCHAIN_HAS_LIBCXX

This commit introduces `TOOLCHAIN_HAS_LIBCXX` CMake variable, which is set
to `y` when LLVM C++ Standard Library aka. libc++ is available.

This helps filter libc++-specific Kconfig and tests in a more refined
manner.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>