target/nios2: Rewrite interrupt handling

target/nios2: Rewrite interrupt handling

Previously, we would avoid setting CPU_INTERRUPT_HARD when interrupts
are disabled at a particular point in time, instead queuing the value
into cpu->irq_pending.  This is more complicated than required.

Instead, set CPU_INTERRUPT_HARD any time there is a pending interrupt,
and exclusively check for interrupts disabled in nios2_cpu_exec_interrupt.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>