runtime: preserve DX during racefuncenter

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/7382049

src/pkg/runtime/race.c

@@ -87,10 +87,10 @@ runtime·raceread(uintptr addr)
 	}
 }
 
-// Called from instrumented code.
+// Called from runtime·racefuncenter (assembly).
 #pragma textflag 7
 void
-runtime·racefuncenter(uintptr pc)
+runtime·racefuncenter1(uintptr pc)
 {
 	// If the caller PC is lessstack, use slower runtime·callers
 	// to walk across the stack split to find the real caller.

src/pkg/runtime/race_amd64.s

@@ -0,0 +1,11 @@
+// Copyright 2013 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.
+
+// +build race
+
+TEXT	runtime·racefuncenter(SB),7,$0
+	PUSHQ	DX // save function entry context (for closures)
+	CALL	runtime·racefuncenter1(SB)
+	POPQ	DX
+	RET