target/arm: Restore M-profile CONTROL.SPSEL before any tailchaining

target/arm: Restore M-profile CONTROL.SPSEL before any tailchaining

On exception return for M-profile, we must restore the CONTROL.SPSEL
bit from the EXCRET value before we do any kind of tailchaining,
including for the derived exceptions on integrity check failures.
Otherwise we will give the guest an incorrect EXCRET.SPSEL value on
exception entry for the tailchained exception.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180720145647.8810-4-peter.maydell@linaro.org