targets/gba: implement interrupt handler

Thanks to Kyle Lemons for the inspiration and original design. The
implementation in this commit is very different however, building on top
of the software vectoring needed in RISC-V. The result is a flexible
interrupt handler that does not take up any RAM for configuration.

src/machine/machine_gameboyadvance.go

@@ -8,6 +8,25 @@ import (
 	"unsafe"
 )
 
+// Interrupt numbers as used on the GameBoy Advance. Register them with
+// runtime/interrupt.New.
+const (
+	IRQ_VBLANK  = 0
+	IRQ_HBLANK  = 1
+	IRQ_VCOUNT  = 2
+	IRQ_TIMER0  = 3
+	IRQ_TIMER1  = 4
+	IRQ_TIMER2  = 5
+	IRQ_TIMER3  = 6
+	IRQ_COM     = 7
+	IRQ_DMA0    = 8
+	IRQ_DMA1    = 9
+	IRQ_DMA2    = 10
+	IRQ_DMA3    = 11
+	IRQ_KEYPAD  = 12
+	IRQ_GAMEPAK = 13
+)
+
 // Make it easier to directly write to I/O RAM.
 var ioram = (*[0x400]volatile.Register8)(unsafe.Pointer(uintptr(0x04000000)))