target/arm: SCR_EL3.RW should be treated as 1 if EL2 doesn't support AArch32

target/arm: SCR_EL3.RW should be treated as 1 if EL2 doesn't support AArch32

The definition of SCR_EL3.RW says that its effective value is 1 if:
 - EL2 is implemented and does not support AArch32, and SCR_EL3.NS is 1
 - the effective value of SCR_EL3.{EEL2,NS} is {1,0} (i.e. we are
   Secure and Secure EL2 is disabled)

We implement the second of these in arm_el_is_aa64(), but forgot the
first.

Provide a new function arm_scr_rw_eff() to return the effective
value of SCR_EL3.RW, and use it in arm_el_is_aa64() and the other
places that currently look directly at the bit value.

(scr_write() enforces that the RW bit is RAO/WI if neither EL1 nor
EL2 have AArch32 support, but if EL1 does but EL2 does not then the
bit must still be writeable.)

This will mean that if code at EL3 attempts to perform an exception
return to AArch32 EL2 when EL2 is AArch64-only we will correctly
handle this as an illegal exception return: it will be caught by the
"return to an EL which is configured for a different register width"
check in HELPER(exception_return).

We do already have some CPU types which don't implement AArch32
above EL0, so this is technically a bug; it doesn't seem worth
backporting to stable because no sensible guest code will be
deliberately attempting to set the RW bit to a value corresponding
to an unimplemented execution state and then checking that we
did the right thing.

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