cmd/8g, cmd/dist, cmd/gc: fix warnings on Plan 9

cmd/8g/gsubr.c: unreachable code
cmd/8g/reg.c: overspecifed class
cmd/dist/plan9.c: unused parameter
cmd/gc/fmt.c: stkdelta is now a vlong
cmd/gc/racewalk.c: used but not set

R=golang-dev, seed, rsc
CC=golang-dev
https://golang.org/cl/7067052

src/cmd/8g/gsubr.c

@@ -1515,7 +1515,7 @@ floatmove(Node *f, Node *t)
 {
 	Node r1, r2, t1, t2, tlo, thi, con, f0, f1, ax, dx, cx;
 	Type *cvt;
-	int a, ft, tt;
+	int ft, tt;
 	Prog *p1, *p2, *p3;
 
 	ft = simsimtype(f->type);
@@ -1711,9 +1711,6 @@ floatmove(Node *f, Node *t)
 		return;
 	}
 
-	gins(a, f, t);
-	return;
-
 hard:
 	// requires register intermediate
 	regalloc(&r1, cvt, t);

src/cmd/8g/reg.c

@@ -1815,7 +1815,7 @@ hash32to16(uint32 h)
  * sees, which lets it do a better job and makes it less likely to turn
  * itself off.
  */
-void
+static void
 fixtemp(Prog *firstp)
 {
 	static uint8 counts[1<<16]; // A hash table to count variable occurences.

src/cmd/dist/plan9.c

@@ -755,6 +755,7 @@ xsamefile(char *f1, char *f2)
 int
 xtryexecfunc(void (*f)(void))
 {
+	USED(f);
 	return 0; // suffice for now
 }
 

src/cmd/gc/fmt.c

@@ -289,7 +289,7 @@ Jconv(Fmt *fp)
 		fmtprint(fp, " l(%d)", n->lineno);
 
 	if(!c && n->xoffset != BADWIDTH)
-		fmtprint(fp, " x(%lld%+d)", n->xoffset, n->stkdelta);
+		fmtprint(fp, " x(%lld%+lld)", n->xoffset, n->stkdelta);
 
 	if(n->class != 0) {
 		s = "";

src/cmd/gc/racewalk.c

@@ -219,6 +219,7 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 		racewalknode(&n->left, init, 0, 0);
 		if(istype(n->left->type, TMAP)) {
 			// crashes on len(m[0]) or len(f())
+			SET(n1);
 			USED(n1);
 			/*
 			n1 = nod(OADDR, n->left, N);