target/avr: Only execute one interrupt at a time

target/avr: Only execute one interrupt at a time

We cannot deliver two interrupts simultaneously;
the first interrupt handler must execute first.

Reviewed-by: Michael Rolnik <mrolnik@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>