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.

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.
+}