target-arm: Restrict check_ap() use of S and R bits to v6 and earlier

The SCTLR bits S and R (8 and 9) only exist in ARMv6 and earlier.
In ARMv7 these bits RAZ, and in ARMv8 they are reassigned. Guard
the use of them in check_ap() so that we don't get incorrect results
for ARMv8 CPUs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
