target-arm: Implement 'int' loglevel

The 'int' loglevel for recording interrupts and exceptions
requires support in the target-specific code. Implement
it for ARM. This improves debug logging in some situations
that were otherwise pretty opaque, such as when we fault
trying to execute at an exception vector address, which
would otherwise cause an infinite loop of taking exceptions
without any indication in the debug log of what was going on.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 1375700771-21665-1-git-send-email-peter.maydell@linaro.org
