Add system-stdc++ for shared libstdc++ linking

[keith]

Depending on your project statically linking libstdc++ from your sysroot might not be viable. For example if you rely on other shared libraries that link the shared version, linking the static version to a binary might result in ODR violations. This new system-stdc++ sets the compile flags correctly but doesn't force the static version, so the sysroot's shared version will be preferred.