target-arm: Forbid mode switch to Mon from Secure EL1

In v8 trying to switch mode to Mon from Secure EL1 is an
illegal mode switch. (In v7 this is impossible as all secure
modes except User are at EL3.) We can handle this case by
making a switch to Mon valid only if the current EL is 3,
which then gives the correct answer whether EL3 is AArch32
or AArch64.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1455556977-3644-9-git-send-email-peter.maydell@linaro.org
