armv7m: Escalate exceptions to HardFault if necessary

armv7m: Escalate exceptions to HardFault if necessary

The v7M exception architecture requires that if a synchronous
exception cannot be taken immediately (because it is disabled
or at too low a priority) then it should be escalated to
HardFault (and the HardFault exception is then taken).
Implement this escalation logic.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
[PMM: extracted from another patch]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>