target/arm: Adjust sve_exception_el

Check for EL3 before testing CPTR_EL3.EZ.  Return 0 when the exception
should be routed via AdvSIMDFPAccessTrap.  Mirror the structure of
CheckSVEEnabled more closely.

Fixes: 5be5e8ed
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181005175350.30752-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>