Add runtime stubs required for net/http

Continued from #1911.

src/runtime/extern.go

@@ -0,0 +1,5 @@
+package runtime
+
+func Callers(skip int, pc []uintptr) int {
+	return 0
+}

src/runtime/symtab.go

@@ -0,0 +1,20 @@
+package runtime
+
+type Frames struct {
+	//
+}
+
+type Frame struct {
+	Function string
+
+	File string
+	Line int
+}
+
+func CallersFrames(callers []uintptr) *Frames {
+	return nil
+}
+
+func (ci *Frames) Next() (frame Frame, more bool) {
+	return Frame{}, false
+}