build: separate compiler and libs

compiler/internal/lib/runtime/extern.go

@@ -0,0 +1,9 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+func Caller(skip int) (pc uintptr, file string, line int, ok bool) {
+	panic("todo: runtime.Caller")
+}