runtime: include os_*.h in the generated Go structs

Adding the #include to defs.c makes it get processed
by cmd/dist, which writes out Go equivalent for all the
C data structures defined in defs.c.

This in turn makes it necessary to define the Plink type,
used in os_plan9.h, in os_plan9.go. Rename it to _Plink
to avoid being exported.

LGTM=0intro, iant
R=golang-codereviews, iant, 0intro
CC=golang-codereviews, r
https://golang.org/cl/132490043

src/pkg/runtime/defs.c

@@ -13,3 +13,4 @@
 #include "chan.h"
 #include "mprof.h"
 #include "defs_GOOS_GOARCH.h"
+#include "os_GOOS.h"

src/pkg/runtime/os_plan9.go

@@ -26,3 +26,5 @@ func errstr() string
 // The size of the note handler frame varies among architectures,
 // but 512 bytes should be enough for every implementation.
 const stackSystem = 512
+
+type _Plink uintptr

src/pkg/runtime/os_plan9.h

@@ -59,15 +59,15 @@ enum
 };
 
 typedef struct Tos Tos;
-typedef intptr Plink;
+typedef intptr _Plink;
 
 struct Tos {
 	struct			/* Per process profiling */
 	{
-		Plink	*pp;	/* known to be 0(ptr) */
+		_Plink	*pp;	/* known to be 0(ptr) */
-		Plink	*next;	/* known to be 4(ptr) */
+		_Plink	*next;	/* known to be 4(ptr) */
-		Plink	*last;
+		_Plink	*last;
-		Plink	*first;
+		_Plink	*first;
 		uint32	pid;
 		uint32	what;
 	} prof;