target/arm: Detect overflow when calculating next PMU interrupt

In pmccntr_op_finish() and pmevcntr_op_finish() we calculate the next
point at which we will get an overflow and need to fire the PMU
interrupt or set the overflow flag.  We do this by calculating the
number of nanoseconds to the overflow event and then adding it to
qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL).  However, we don't check
whether that signed addition overflows, which can happen if the next
PMU interrupt would happen massively far in the future (250 years or
more).

Since QEMU assumes that "when the QEMU_CLOCK_VIRTUAL rolls over" is
"never", the sensible behaviour in this situation is simply to not
try to set the timer if it would be beyond that point.  Detect the
overflow, and skip setting the timer in that case.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220822132358.3524971-7-peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>