cpu-exec: Do CPU_INTERRUPT_HALT unconditionally

The signal is currently checked by 10 targets, but only actually
raised by Sparc and ARM.  For the sake of one test-and-branch,
we can handle this generic bit generically.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-id: 1410626734-3804-24-git-send-email-rth@twiddle.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>