target/arm: Send interrupts on PMU counter overflow

Whenever we notice that a counter overflow has occurred, send an
interrupt. This is made more reliable with the addition of a timer in a
follow-on commit.

Signed-off-by: Aaron Lindsay <aaron@os.amperecomputing.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190124162401.5111-2-aaron@os.amperecomputing.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>