armv7m: Simpler and faster exception start

armv7m: Simpler and faster exception start

All the places in armv7m_cpu_do_interrupt() which pend an
exception in the NVIC are doing so for synchronous
exceptions. We know that we will always take some
exception in this case, so we can just acknowledge it
immediately, rather than returning and then immediately
being called again because the NVIC has raised its outbound
IRQ line.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
[PMM: tweaked commit message; added DEBUG to the set of
exceptions we handle immediately, since it is synchronous
when it results from the BKPT instruction]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>