armv7m: Fix condition check for taking exceptions

The M profile condition for when we can take a pending exception or
interrupt is not the same as that for A/R profile.  The code
originally copied from the A/R profile version of the
cpu_exec_interrupt function only worked by chance for the
very simple case of exceptions being masked by PRIMASK.
Replace it with a call to a function in the NVIC code that
correctly compares the priority of the pending exception
against the current execution priority of the CPU.

[Michael Davidsaver's patchset had a patch to do something
similar but the implementation ended up being a rewrite.]

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
