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/runtime/interrupt/interrupt_gameboyadvance.go

@@ -0,0 +1,36 @@
+// +build gameboyadvance
+
+package interrupt
+
+import (
+	"runtime/volatile"
+	"unsafe"
+)
+
+var (
+	regInterruptEnable       = (*volatile.Register16)(unsafe.Pointer(uintptr(0x4000200)))
+	regInterruptRequestFlags = (*volatile.Register16)(unsafe.Pointer(uintptr(0x4000202)))
+	regInterruptMasterEnable = (*volatile.Register16)(unsafe.Pointer(uintptr(0x4000208)))
+)
+
+// Enable enables this interrupt. Right after calling this function, the
+// interrupt may be invoked if it was already pending.
+func (irq Interrupt) Enable() {
+	regInterruptEnable.SetBits(1 << uint(irq.num))
+}
+
+//export handleInterrupt
+func handleInterrupt() {
+	flags := regInterruptRequestFlags.Get()
+	for i := 0; i < 14; i++ {
+		if flags&(1<<uint(i)) != 0 {
+			regInterruptRequestFlags.Set(1 << uint(i)) // acknowledge interrupt
+			callInterruptHandler(i)
+		}
+	}
+}
+
+// callInterruptHandler is a compiler-generated function that calls the
+// appropriate interrupt handler for the given interrupt ID.
+//go:linkname callInterruptHandler runtime.callInterruptHandler
+func callInterruptHandler(id int)

src/runtime/runtime_arm7tdmi.go

@@ -3,6 +3,7 @@
 package runtime
 
 import (
+	_ "runtime/interrupt" // make sure the interrupt handler is defined
 	"unsafe"
 )