cmd/5g, cmd/8g: make 'out of registers' a fatal error

There's no point in continuing. We will only get confused.
6g already makes this fatal.

LGTM=dave, minux, iant
R=iant, dave, minux
CC=golang-codereviews
https://golang.org/cl/140660043

src/cmd/5g/gsubr.c

@@ -361,7 +361,7 @@ regalloc(Node *n, Type *t, Node *o)
 		print("registers allocated at\n");
 		for(i=REGALLOC_R0; i<=REGALLOC_RMAX; i++)
 			print("%d %p\n", i, regpc[i]);
-		yyerror("out of fixed registers");
+		fatal("out of fixed registers");
 		goto err;
 
 	case TFLOAT32:
@@ -374,7 +374,7 @@ regalloc(Node *n, Type *t, Node *o)
 		for(i=REGALLOC_F0; i<=REGALLOC_FMAX; i++)
 			if(reg[i] == 0)
 				goto out;
-		yyerror("out of floating point registers");
+		fatal("out of floating point registers");
 		goto err;
 
 	case TCOMPLEX64:

src/cmd/8g/gsubr.c

@@ -938,7 +938,7 @@ regalloc(Node *n, Type *t, Node *o)
 		fprint(2, "registers allocated at\n");
 		for(i=D_AX; i<=D_DI; i++)
 			fprint(2, "\t%R\t%#lux\n", i, regpc[i]);
-		yyerror("out of fixed registers");
+		fatal("out of fixed registers");
 		goto err;
 
 	case TFLOAT32: