target/arm: Implement new v8.1M NOCP check for exception return

target/arm: Implement new v8.1M NOCP check for exception return

In v8.1M a new exception return check is added which may cause a NOCP
UsageFault (see rule R_XLTP): before we clear s0..s15 and the FPSCR
we must check whether access to CP10 from the Security state of the
returning exception is disabled; if it is then we must take a fault.

(Note that for our implementation CPPWR is always RAZ/WI and so can
never cause CP10 accesses to fail.)

The other v8.1M change to this register-clearing code is that if MVE
is implemented VPR must also be cleared, so add a TODO comment to
that effect.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201119215617.29887-20-peter.maydell@linaro.org