i386: implement IGNNE

Change the handling of port F0h writes and FPU exceptions to implement IGNNE.

The implementation mixes a bit what the chipset and processor do in real
hardware, but the effect is the same as what happens with actual FERR#
and IGNNE# pins: writing to port F0h asserts IGNNE# in addition to lowering
FP_IRQ; while clearing the SE bit in the FPU status word deasserts IGNNE#.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/i386/pc.c

@@ -385,7 +385,7 @@ static void ioportF0_write(void *opaque, hwaddr addr, uint64_t data,
                            unsigned size)
 {
     if (tcg_enabled()) {
-        cpu_clear_ferr();
+        cpu_set_ignne();
     }
 }