runtime: implement local work queues (in preparation for new scheduler)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7402047

src/pkg/runtime/runtime.h

diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 2459199..61e33eb 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -52,6 +52,7 @@
 typedef	struct	Gobuf		Gobuf;
 typedef	union	Lock		Lock;
 typedef	struct	M		M;
+typedef struct	P		P;
 typedef	struct	Mem		Mem;
 typedef	union	Note		Note;
 typedef	struct	Slice		Slice;
@@ -312,6 +313,17 @@
 	uintptr	end[];
 };
 
+struct P
+{
+	Lock;
+
+	// Queue of runnable goroutines.
+	G**	runq;
+	int32	runqhead;
+	int32	runqtail;
+	int32	runqsize;
+};
+
 // The m->locked word holds a single bit saying whether
 // external calls to LockOSThread are in effect, and then a counter
 // of the internal nesting depth of lockOSThread / unlockOSThread.