target/arm: Fix output of PAuth Auth

target/arm: Fix output of PAuth Auth

The ARM pseudocode installs the error_code into the original
pointer, not the encrypted pointer.  The difference applies
within the 7 bits of pac data; the result should be the sign
extension of bit 55.

Add a testcase to that effect.

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