fix gc bug causing make smoketest to die in cmd/gofmt.

saving of sp was too far away from use in scanstack;
the stack had changed since the sp was saved.

R=r
DELTA=9  (4 added, 2 deleted, 3 changed)
OCL=32232
CL=32237

src/pkg/runtime/amd64/traceback.c

@@ -26,6 +26,7 @@ traceback(byte *pc0, byte *sp, G *g)
 	for(n=0; n<100; n++) {
 		if(pc == (uint64)sys·lessstack) {
 			// pop to earlier stack block
+			// printf("-- stack jump %p => %p\n", sp, stk->gobuf.sp);
 			pc = (uintptr)stk->gobuf.pc;
 			sp = stk->gobuf.sp;
 			stk = (Stktop*)stk->stackbase;

src/pkg/runtime/mgc0.c

@@ -61,13 +61,16 @@ scanblock(int32 depth, byte *b, int64 n)
 }
 
 static void
-scanstack(G *g)
+scanstack(G *gp)
 {
 	Stktop *stk;
 	byte *sp;
 
-	sp = g->sched.sp;
-	stk = (Stktop*)g->stackbase;
+	if(gp == g)
+		sp = (byte*)&gp;
+	else
+		sp = gp->sched.sp;
+	stk = (Stktop*)gp->stackbase;
 	while(stk) {
 		scanblock(0, sp, (byte*)stk - sp);
 		sp = stk->gobuf.sp;
@@ -220,7 +223,6 @@ gc(int32 force)
 //printf("gc...\n");
 	m->gcing = 1;
 	semacquire(&gcsema);
-	gosave(&g->sched);	// update g's stack pointer for scanstack
 	stoptheworld();
 	if(mheap.Lock.key != 0)
 		throw("mheap locked during gc");
@@ -230,7 +232,6 @@ gc(int32 force)
 		mstats.next_gc = mstats.inuse_pages+mstats.inuse_pages*gcpercent/100;
 	}
 	starttheworld();
-	gosave(&g->sched);	// update g's stack pointer for debugging
 	semrelease(&gcsema);
 	m->gcing = 0;
 }