target/arm: Copy guarded bit in combine_cacheattrs

The guarded bit comes from the stage1 walk.

Fixes: Coverity CID 1507929
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230407185149.3253946-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>