target/arm: Implement FEAT_S2FWB

Implement the handling of FEAT_S2FWB; the meat of this is in the new
combined_attrs_fwb() function which combines S1 and S2 attributes
when HCR_EL2.FWB is set.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220505183950.2781801-4-peter.maydell@linaro.org