target/arm: Use bit 55 explicitly for pauth

target/arm: Use bit 55 explicitly for pauth

The psuedocode in aarch64/functions/pac/auth/Auth and
aarch64/functions/pac/strip/Strip always uses bit 55 for
extfield and do not consider if the current regime has 2 ranges.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200216194343.21331-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>