cpu-exec.c: Allow disabling of IRQs on ARM Cortex-M CPUs

cpu-exec.c: Allow disabling of IRQs on ARM Cortex-M CPUs

Correct an error in the logic for deciding whether we can
take an IRQ interrupt which meant that on M profile cores
it was never possible to disable them.

The design here is still bogus in that M profile doesn't
have separate "IRQ" and "FIQ", which are an A/R profile
concept; we should ideally implement the proper priority
based scheme.

Signed-off-by: David Hoover <spm@boiteauxlettres.sent.at>
[PMM: Wrote a proper commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>