cmake: toolchain: Introduce TOOLCHAIN_HAS_GLIBCXX

This commit introduces `TOOLCHAIN_HAS_GLIBCXX` CMake variable, which is
set to `y` when GNU C++ Standard Library aka. libstdc++ is available.

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

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