gba: add bios interrupt flags

[zowhoey]

Adds a new GBA register for interrupt flags, the register is equivalent
to the IF register, but is intended for BIOS functions.

Acknowledging the interrupts with this register allows us to use BIOS
halt functions such as VBlankIntrWait and IntrWait, which we can use to
get rid of busy loops in the GBA code.

Example code:

func vblank(interrupt.Interrupt) {}

func main() {
        // enable vblank interrupt
        gba.DISP.DISPSTAT.ReplaceBits(1, 1, 3)
        // setup a vblank handler
        interrupt.New(machine.IRQ_VBLANK, vblank).Enable()
        for {
                if gba.DISP.VCOUNT.Get() > 160 {
                        println("vblanking")
                } else {
                        println("busy loop")
                }

                // uncomment to get rid of the busy loop
                // call VBlankIntrWait
                // arm.SVCall0(0x05 << 16)
        }
}

[zowhoey]

Related question: if I were to implement some of the BIOS functions should they go into tinygba or into the device/gba package? Having it in device/gba might be nice as we could use the BIOS halt function in func abort() in the runtime code.

[deadprogram]

if I were to implement some of the BIOS functions should they go into tinygba or into the device/gba package? Having it in device/gba might be nice as we could use the BIOS halt function in func abort() in the runtime code.

Probably in device/gba since they are the actual ROM functions. tinygba tries to provide "tools and helpers".

[deadprogram]

Unformatted:
  src/runtime/interrupt/interrupt_gameboyadvance.go
make: *** [GNUmakefile:213: fmt-check] Error 1

Please run go fmt so this PR can pass the CI tests. Thanks!

[zowhoey]

Woops! I've disabled auto-formatting in my IDE for tinygo as my usual formatting has a lot more rules and was affecting random code unrelated to the change.

[deadprogram]

I really wanted this myself, pretty excited to get rid of the looping!

[deadprogram]

OK thank you very much @zowhoey now squash/merging.