target/arm: PSTATE.PAN should not clear exec bits

target/arm: PSTATE.PAN should not clear exec bits

Our implementation of the PSTATE.PAN bit incorrectly cleared all
access permission bits for privileged access to memory which is
user-accessible.  It should only affect the privileged read and write
permissions; execute permission is dealt with via XN/PXN instead.

Fixes: 81636b70c226dc27d7ebc8d
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200330170651.20901-1-peter.maydell@linaro.org