Currently `rules_rust` won't link against the C++ standard library in case
C++ toolchain is the `static_link_cpp_runtimes` feature (therefore when
it uses `cc_toolchain.static_runtime_lib` and `cc_toolchain.dynamic_runtime_lib`
attributes to declare which version of C++ static library should be
linked into the final binary).

This PR modifies `get_libs_for_static_executable` to also look into the
C++ toolchain and depend on C++ standard library from there.

Once we implement the support for dynamically linked rust binaries we
will likely have an equivalend `get_libs_for_dynamic_executable`, which
would then look into `cc_toolchain.dynamic_runtime_lib`.

While there, I moved the `get_cc_toolchain` function to the `utils.rs`,
to be consistent with the handling of the Rust toolchain.