cmd/ld: fix compilation when GOARCH != GOHOSTARCH

R=rsc, dave, minux.ma
CC=golang-dev
https://golang.org/cl/6493123

src/pkg/runtime/mgc0.c

@@ -11,7 +11,6 @@
 
 enum {
 	Debug = 0,
-	PtrSize = sizeof(void*),
 	DebugMark = 0,  // run second pass to check mark
 	DataBlock = 8*1024,
 

src/pkg/runtime/runtime.h

@@ -118,6 +118,10 @@ enum
 	true	= 1,
 	false	= 0,
 };
+enum
+{
+	PtrSize = sizeof(void*),
+};
 
 /*
  * structures

src/pkg/runtime/typekind.h

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// PtrSize vs sizeof(void*): This file is also included from src/cmd/ld/...
+// which defines PtrSize to be different from sizeof(void*) when crosscompiling.
+
 enum {
 	KindBool = 1,
 	KindInt,
@@ -33,6 +36,6 @@ enum {
 	KindNoPointers = 1<<7,
 
 	// size of Type interface header + CommonType structure.
-	CommonSize = 2*sizeof(void*) + 6*sizeof(void*) + 8,
+	CommonSize = 2*PtrSize + 6*PtrSize + 8,
 };