runtime: do not unset the special bit after finalization.

A block with finalizer might also be profiled. The special bit
is needed to unregister the block from the profile. It will be
unset only when the block is freed.

Fixes #3668.

R=golang-dev, rsc
CC=golang-dev, remy
https://golang.org/cl/6249066

src/pkg/runtime/mfinal.c

@@ -150,8 +150,7 @@ runtime·addfinalizer(void *p, void (*f)(void*), int32 nret)
 	tab = TAB(p);
 	runtime·lock(tab);
 	if(f == nil) {
-		if(lookfintab(tab, p, true, nil))
-			runtime·setblockspecial(p, false);
+		lookfintab(tab, p, true, nil);
 		runtime·unlock(tab);
 		return true;
 	}

src/pkg/runtime/mgc0.c

@@ -1042,7 +1042,6 @@ runfinq(void)
 					framecap = framesz;
 				}
 				*(void**)frame = f->arg;
-				runtime·setblockspecial(f->arg, false);
 				reflect·call((byte*)f->fn, frame, sizeof(uintptr) + f->nret);
 				f->fn = nil;
 				f->arg = nil;