target/arm: Don't set ISV when reporting stage 1 faults in ESR_EL2

target/arm: Don't set ISV when reporting stage 1 faults in ESR_EL2

The syndrome value reported to ESR_EL2 should only contain the
detailed instruction syndrome information when the fault has been
caused by a stage 2 abort, not when the fault was a stage 1 abort
(i.e.  caused by execution at EL2).  We were getting this wrong and
reporting the detailed ISV information all the time.

Fix the bug by checking fi->stage2.  Add a TODO comment noting the
cases where we'll have to come back and revisit this when we
implement FEAT_LS64 and friends.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230331145045.2584941-3-peter.maydell@linaro.org