target/arm: Make sure M-profile FPSCR RES0 bits are not settable

target/arm: Make sure M-profile FPSCR RES0 bits are not settable

Enforce that for M-profile various FPSCR bits which are RES0 there
but have defined meanings on A-profile are never settable. This
ensures that M-profile code can't enable the A-profile behaviour
(notably vector length/stride handling) by accident.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190416125744.27770-2-peter.maydell@linaro.org