nvic: Handle v8M changes in nvic_exec_prio()

Update nvic_exec_prio() to support the v8M changes:
 * BASEPRI, FAULTMASK and PRIMASK are all banked
 * AIRCR.PRIS can affect NS priorities
 * AIRCR.BFHFNMINS affects FAULTMASK behaviour

These changes mean that it's no longer possible to
definitely say that if FAULTMASK is set it overrides
PRIMASK, and if PRIMASK is set it overrides BASEPRI
(since if PRIMASK_NS is set and AIRCR.PRIS is set then
whether that 0x80 priority should take effect or the
priority in BASEPRI_S depends on the value of BASEPRI_S,
for instance). So we switch to the same approach used
by the pseudocode of working through BASEPRI, PRIMASK
and FAULTMASK and overriding the previous values if
needed.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1505240046-11454-16-git-send-email-peter.maydell@linaro.org
