targets: add support for GameBoy Advance

Only tested in an emulator (mGBA). Almost nothing is supported, but
drawing to the screen works.

src/runtime/runtime_arm7tdmi.go

@@ -0,0 +1,101 @@
+// +build arm7tdmi
+
+package runtime
+
+import (
+	"device/arm"
+	"unsafe"
+)
+
+const TargetBits = 32
+
+const GOARCH = "arm"
+
+type timeUnit int64
+
+const tickMicros = 1
+
+func putchar(c byte) {
+	// dummy, TODO
+}
+
+//go:extern _sbss
+var _sbss unsafe.Pointer
+
+//go:extern _ebss
+var _ebss unsafe.Pointer
+
+//go:extern _sdata
+var _sdata unsafe.Pointer
+
+//go:extern _sidata
+var _sidata unsafe.Pointer
+
+//go:extern _edata
+var _edata unsafe.Pointer
+
+// Entry point for Go. Initialize all packages and call main.main().
+//go:export main
+func main() {
+	// Initialize .data and .bss sections.
+	preinit()
+
+	// Run initializers of all packages.
+	initAll()
+
+	// Compiler-generated call to main.main().
+	callMain()
+}
+
+func preinit() {
+	// Initialize .bss: zero-initialized global variables.
+	ptr := unsafe.Pointer(&_sbss)
+	for ptr != unsafe.Pointer(&_ebss) {
+		*(*uint32)(ptr) = 0
+		ptr = unsafe.Pointer(uintptr(ptr) + 4)
+	}
+
+	// Initialize .data: global variables initialized from flash.
+	src := unsafe.Pointer(&_sidata)
+	dst := unsafe.Pointer(&_sdata)
+	for dst != unsafe.Pointer(&_edata) {
+		*(*uint32)(dst) = *(*uint32)(src)
+		dst = unsafe.Pointer(uintptr(dst) + 4)
+		src = unsafe.Pointer(uintptr(src) + 4)
+	}
+}
+
+func ticks() timeUnit {
+	// TODO
+	return 0
+}
+
+const asyncScheduler = false
+
+func sleepTicks(d timeUnit) {
+	// TODO
+}
+
+func abort() {
+	// TODO
+	for {
+	}
+}
+
+func getCurrentStackPointer() uintptr {
+	return arm.ReadRegister("sp")
+}
+
+// Implement memset for LLVM and compiler-rt.
+//go:export memset
+func libc_memset(ptr unsafe.Pointer, c byte, size uintptr) {
+	for i := uintptr(0); i < size; i++ {
+		*(*byte)(unsafe.Pointer(uintptr(ptr) + i)) = c
+	}
+}
+
+// Implement memmove for LLVM and compiler-rt.
+//go:export memmove
+func libc_memmove(dst, src unsafe.Pointer, size uintptr) {
+	memmove(dst, src, size)
+}