bazel config "clang" and "clang_san" has no associated clang toolchain

Currently we have no clang toolchain in place. Both bazel configs clang and clang_san are falling back to gcc (as clang is not in place). See https://gitlab.eclipse.org/eclipse/openpass/gt-gen-core/-/blob/2a32599288405119deceda533c7e94c45fc22fb6/.bazelrc#L50-L67