runtime/cgo: use just #include "textflag.h"

This will withstand the src/pkg/ to src/ move.

LGTM=r
R=iant, r
CC=golang-codereviews
https://golang.org/cl/134530044

src/pkg/runtime/cgo/asm_386.s

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#include "../../../cmd/ld/textflag.h"
+#include "textflag.h"
 
 /*
  * void crosscall2(void (*fn)(void*, int32), void*, int32)

src/pkg/runtime/cgo/asm_amd64.s

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#include "../../../cmd/ld/textflag.h"
+#include "textflag.h"
 
 /*
  * void crosscall2(void (*fn)(void*, int32), void*, int32)

src/pkg/runtime/cgo/asm_arm.s

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#include "../../../cmd/ld/textflag.h"
+#include "textflag.h"
 
 /*
  * void crosscall2(void (*fn)(void*, int32), void*, int32)

src/pkg/runtime/cgo/asm_nacl_amd64p32.s

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-#include "../../../cmd/ld/textflag.h"
+#include "textflag.h"
 
 /*
  * void crosscall2(void (*fn)(void*, int32), void*, int32)

src/pkg/runtime/cgo/callbacks.c

@@ -4,7 +4,7 @@
 
 #include "../runtime.h"
 #include "../cgocall.h"
-#include "../../../cmd/ld/textflag.h"
+#include "textflag.h"
 
 // These utility functions are available to be called from code
 // compiled with gcc via crosscall2.