compiler: rewrite defer support to better support them

This costs a bit in code size, but should be more flexible for the
future to implement recover() and running deferred functions while
panicking.

src/runtime/defer.go

@@ -0,0 +1,17 @@
+package runtime
+
+import "unsafe"
+
+type deferContext unsafe.Pointer
+
+type _defer struct {
+	callback func(*_defer)
+	next     *_defer
+}
+
+func rundefers(stack *_defer) {
+	for stack != nil {
+		stack.callback(stack)
+		stack = stack.next
+	}
+}