build: add intptr typedef for Plan 9

Revision 01810e5c68e9 added the following to src/pkg/runtime/stack.h:

        StackPreempt = (uintptr)(intptr)0xffffade,

The typedef for intptr is defined in two places:

        1. src/pkg/runtime/runtime.h for use by the runtime

        2. include/u.h for use by the compilers and linkers

On Plan 9, we don't use include/u.h but instead augment the host's
u.h with extra typedefs. These are in include/plan9/GOARCH/u.h.

We forgot to add intptr. It didn't cause a problem until now since
that typedef was never used outside the runtime.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10023043

include/plan9/386/u.h

@@ -4,11 +4,12 @@
 
 #include "/386/include/u.h"
 
-typedef char   int8;
-typedef uchar  uint8;
-typedef short  int16;
-typedef ushort uint16;
-typedef int    int32;
-typedef uint   uint32;
-typedef vlong  int64;
-typedef uvlong uint64;
+typedef	char	int8;
+typedef	uchar	uint8;
+typedef	short	int16;
+typedef	ushort	uint16;
+typedef	int	int32;
+typedef	uint	uint32;
+typedef	vlong	int64;
+typedef	uvlong	uint64;
+typedef	int	intptr;

include/plan9/amd64/u.h

@@ -4,11 +4,12 @@
 
 #include "/amd64/include/u.h"
 
-typedef char   int8;
-typedef uchar  uint8;
-typedef short  int16;
-typedef ushort uint16;
-typedef int    int32;
-typedef uint   uint32;
-typedef vlong  int64;
-typedef uvlong uint64;
+typedef	char	int8;
+typedef	uchar	uint8;
+typedef	short	int16;
+typedef	ushort	uint16;
+typedef	int	int32;
+typedef	uint	uint32;
+typedef	vlong	int64;
+typedef	uvlong	uint64;
+typedef	vlong	intptr;

include/plan9/arm/u.h

@@ -4,11 +4,12 @@
 
 #include "/arm/include/u.h"
 
-typedef char   int8;
-typedef uchar  uint8;
-typedef short  int16;
-typedef ushort uint16;
-typedef int    int32;
-typedef uint   uint32;
-typedef vlong  int64;
-typedef uvlong uint64;
+typedef	char	int8;
+typedef	uchar	uint8;
+typedef	short	int16;
+typedef	ushort	uint16;
+typedef	int	int32;
+typedef	uint	uint32;
+typedef	vlong	int64;
+typedef	uvlong	uint64;
+typedef	int	intptr;