target-arm: Implement checking of fired watchpoint

ARM stops before access to a location covered by watchpoint. Also, QEMU
watchpoint fire is not necessarily an architectural watchpoint match.
Unfortunately, that is hardly possible to ignore a fired watchpoint in
debug exception handler. So move watchpoint check from debug exception
handler to the dedicated watchpoint checking callback.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1454256948-10485-3-git-send-email-serge.fdrv@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
