system: check reset button state correctly on gamecube

[DacoTaco]

also unmask the irq after callback so pressing it again re-triggers the irq

Things to note :

• the registry always contains the state of the reset button ( 0 = pressed, 1 = unpressed ) and the IRQ only triggers when going from 1 to 0 (it doesn't trigger when holding it, or releasing it)
• the while loop i left as is, besides it now checking on reset not being pressed (to deal with debounce)
• callback is executed when the button is pressed (unlike before where it only executed if it wasn't pressed)
• IRQ is unmasked after the callback so the IRQ can be triggered again ( and therefor acts the same on GC and Wii )