target-arm: Avoid "1 << 31" undefined behaviour

Avoid the undefined behaviour of "1 << 31" by using 1U to make
the shift be of an unsigned value rather than shifting into the
sign bit of a signed integer. For consistency, we make all the
CPSR_* constants unsigned, though the only one which triggers
undefined behaviour is CPSR_N.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 1378391908-22137-3-git-send-email-peter.maydell@linaro.org
