runtime: eliminate false sharing on runtime.goidgen

runtime.goidgen can be quite frequently modified and
shares cache line with the following variables,
it leads to false sharing.
50c6b0 b nfname
50c6b4 b nfunc
50c6b8 b nfunc$17
50c6bc b nhist$17
50c6c0 B runtime.checking
50c6c4 B runtime.gcwaiting
50c6c8 B runtime.goidgen
50c6cc B runtime.gomaxprocs
50c6d0 B runtime.panicking
50c6d4 B strconv.IntSize
50c6d8 B src/pkg/runtime/_xtest_.ss
50c6e0 B src/pkg/runtime/_xtest_.stop
50c6e8 b addrfree
50c6f0 b addrmem
50c6f8 b argv

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4673054

src/pkg/runtime/proc.c

@@ -56,6 +56,7 @@ struct Sched {
 	Lock;
 
 	G *gfree;	// available gs (status == Gdead)
+	int32 goidgen;
 
 	G *ghead;	// gs waiting to run
 	G *gtail;
@@ -907,8 +908,8 @@ runtime·newproc1(byte *fn, byte *argp, int32 narg, int32 nret, void *callerpc)
 	newg->gopc = (uintptr)callerpc;
 
 	runtime·sched.gcount++;
-	runtime·goidgen++;
-	newg->goid = runtime·goidgen;
+	runtime·sched.goidgen++;
+	newg->goid = runtime·sched.goidgen;
 
 	newprocreadylocked(newg);
 	schedunlock();

src/pkg/runtime/runtime.h

@@ -369,7 +369,6 @@ extern	Alg	runtime·algarray[Amax];
 extern	String	runtime·emptystring;
 G*	runtime·allg;
 M*	runtime·allm;
-int32	runtime·goidgen;
 extern	int32	runtime·gomaxprocs;
 extern	uint32	runtime·panicking;
 extern	int32	runtime·gcwaiting;		// gc is waiting to run