target/arm: Factor out FWB=0 specific part of combine_cacheattrs()

Factor out the part of combine_cacheattrs() that is specific to
handling HCR_EL2.FWB == 0.  This is the part where we combine the
memory type and cacheability attributes.

The "force Outer Shareable for Device or Normal Inner-NC Outer-NC"
logic remains in combine_cacheattrs() because it holds regardless
(this is the equivalent of the pseudocode EffectiveShareability()
function).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220505183950.2781801-3-peter.maydell@linaro.org