runtime: add "none" garbage collector

This collector does not implement runtime.alloc, so it is a
quick-and-dirty way to check where in a program memory is allocated.

main.go

@@ -435,7 +435,7 @@ func handleCompilerError(err error) {
 func main() {
 	outpath := flag.String("o", "", "output filename")
 	opt := flag.String("opt", "z", "optimization level: 0, 1, 2, s, z")
-	gc := flag.String("gc", "dumb", "garbage collector to use (dumb)")
+	gc := flag.String("gc", "dumb", "garbage collector to use (none, dumb)")
 	printIR := flag.Bool("printir", false, "print LLVM IR")
 	dumpSSA := flag.Bool("dumpssa", false, "dump internal Go SSA")
 	target := flag.String("target", "", "LLVM target")

src/runtime/gc_none.go

@@ -0,0 +1,29 @@
+// +build gc.none
+
+package runtime
+
+// This GC strategy provides no memory allocation at all. It can be useful to
+// detect where in a program memory is allocated, or to combine this runtime
+// with a separate (external) garbage collector.
+
+import (
+	"unsafe"
+)
+
+func alloc(size uintptr) unsafe.Pointer
+
+func free(ptr unsafe.Pointer) {
+	// Nothing to free when nothing gets allocated.
+}
+
+func GC() {
+	// Unimplemented.
+}
+
+func KeepAlive(x interface{}) {
+	// Unimplemented. Only required with SetFinalizer().
+}
+
+func SetFinalizer(obj interface{}, finalizer interface{}) {
+	// Unimplemented.
+}