cmd/gc: fix build for 'default unsigned char' compilers

TBR=iant
CC=golang-codereviews
https://golang.org/cl/63680045

src/cmd/gc/plive.c

@@ -521,7 +521,7 @@ newcfg(Prog *firstp)
 
 			// Stop before an unreachable RET, to avoid creating
 			// unreachable control flow nodes.
-			if(p->link != nil && p->link->as == ARET && p->link->mode == -1)
+			if(p->link != nil && p->link->as == ARET && p->link->mode == 1)
 				break;
 
 			// Collect basic blocks with selectgo calls.

src/cmd/gc/popt.c

@@ -155,7 +155,7 @@ fixjmp(Prog *firstp)
 				// this assumption will not hold in the case of an infinite loop
 				// at the end of a function.
 				// Keep the RET but mark it dead for the liveness analysis.
-				p->mode = -1;
+				p->mode = 1;
 			} else {
 				if(debug['R'] && debug['v'])
 					print("del %P\n", p);