gc: fix error for floating-point constant %

R=ken2
CC=golang-dev
https://golang.org/cl/5674108

src/cmd/gc/const.c

@@ -660,6 +660,14 @@ evconst(Node *n)
 		}
 		mpdivfltflt(v.u.fval, rv.u.fval);
 		break;
+	case TUP(OMOD, CTFLT):
+		// The default case above would print 'ideal % ideal',
+		// which is not quite an ideal error.
+		if(!n->diag) {
+			yyerror("illegal constant expression: floating-point %% operation");
+			n->diag = 1;
+		}
+		return;
 	case TUP(OADD, CTCPLX):
 		mpaddfltflt(&v.u.cval->real, &rv.u.cval->real);
 		mpaddfltflt(&v.u.cval->imag, &rv.u.cval->imag);

test/const1.go

@@ -64,6 +64,7 @@ var (
 	c3 float64 = float64(Big) * Big // ERROR "overflow"
 	c4         = Big * Big          // ERROR "overflow"
 	c5         = Big / 0            // ERROR "division by zero"
+	c6         = 1000 % 1e3         // ERROR "floating-point % operation"
 )
 
 func f(int)