runtime: move gostringw and gogobytes test stubs to Go.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/141150043

src/pkg/runtime/export_test.go

@@ -142,10 +142,19 @@ var MemclrBytes = memclrBytes
 
 var HashLoad = &hashLoad
 
-func gogoBytes() int32
-
-var GogoBytes = gogoBytes
+// For testing.
+func GogoBytes() int32 {
+	return _RuntimeGogoBytes
+}
 
-func gostringW([]uint16) string
+// in string.c
+//go:noescape
+func gostringw(w *uint16) string
 
-var GostringW = gostringW
+// entry point for testing
+func GostringW(w []uint16) (s string) {
+	onM(func() {
+		s = gostringw(&w[0])
+	})
+	return
+}

src/pkg/runtime/stubs.goc

@@ -23,12 +23,6 @@ package runtime
 // These invariants do not hold yet but will be established once we have
 // finished converting runtime support code from C to Go.
 
-// entry point for testing
-// TODO: mcall and run on M stack
-func gostringW(str Slice) (s String) {
-	s = runtime·gostringw((uint16*)str.array);
-}
-
 #pragma textflag NOSPLIT
 func runtime·getg() (ret *G) {
 	ret = g;
@@ -61,8 +55,3 @@ func reflect·typelinks() (ret Slice) {
         ret.len = runtime·etypelink - runtime·typelink;
         ret.cap = ret.len;
 }
-
-// For testing.
-func gogoBytes() (x int32) {
-	x = RuntimeGogoBytes;
-}