target/arm: When tag memory is not present, set MTE=1

When the cpu support MTE, but the system does not, reduce cpu
support to user instructions at EL0 instead of completely
disabling MTE.  If we encounter a cpu implementation which does
something else, we can revisit this setting.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20230811214031.171020-5-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>