target/arm: Update MSR access for PAN

target/arm: Update MSR access for PAN

For aarch64, there's a dedicated msr (imm, reg) insn.
For aarch32, this is done via msr to cpsr.  Writes from el0
are ignored, which is already handled by the CPSR_USER mask.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200208125816.14954-12-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>