target/arm: Forbid return to AArch32 when CPU is AArch64-only

target/arm: Forbid return to AArch32 when CPU is AArch64-only

In the Arm ARM, rule R_TYTWB states that returning to AArch32
is an illegal exception return if:
 * AArch32 is not supported at any exception level
 * the target EL is configured for AArch64 via SCR_EL3.RW
   or HCR_EL2.RW or via CPU state at reset

We check the second of these, but not the first (which can only be
relevant for the case of a return to EL0, because if AArch32 is not
supported at one of the higher ELs then the RW bits will have an
effective value of 1 and the the "configured for AArch64" condition
will hold also).

Add the missing condition. Although this is technically a bug
(because we have one AArch64-only CPU: a64fx) it isn't worth
backporting to stable because no sensible guest code will
deliberately try to return to a nonexistent execution state
to check that it gets an illegal exception return.

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