target/arm: Fix MTE access checks for disabled SEL2

target/arm: Fix MTE access checks for disabled SEL2

While not mentioned anywhere in the actual specification text, the
HCR_EL2.ATA bit is treated as '1' when EL2 is disabled at the current
security state. This can be observed in the psuedo-code implementation
of AArch64.AllocationTagAccessIsEnabled().

Signed-off-by: Idan Horowitz <idan.horowitz@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220328173107.311267-1-idan.horowitz@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>