linux-user: Use restrictive mask when calling cpsr_write()

When linux-user code is calling cpsr_write(), use a restrictive
mask to ensure we are limiting the set of CPSR bits we update.
In particular, don't allow the mode bits to be changed.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1455556977-3644-5-git-send-email-peter.maydell@linaro.org
