math/big, cmd/internal/gc/big: fix vet detected printf problem

Change-Id: I54425d8cbe0277d7a0c9d66c37f2128a0dfa6441
Reviewed-on: https://go-review.googlesource.com/10041
Run-TryBot: Minux Ma <minux@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/internal/gc/big/float_test.go

@@ -1656,7 +1656,7 @@ func TestFloatCmpSpecialValues(t *testing.T) {
 					want = +1
 				}
 				if got != want {
-					t.Errorf("(%g).Cmp(%g) = %s; want %s", x, y, got, want)
+					t.Errorf("(%g).Cmp(%g) = %v; want %v", x, y, got, want)
 				}
 			}
 		}

src/math/big/float_test.go

@@ -1659,7 +1659,7 @@ func TestFloatCmpSpecialValues(t *testing.T) {
 					want = +1
 				}
 				if got != want {
-					t.Errorf("(%g).Cmp(%g) = %s; want %s", x, y, got, want)
+					t.Errorf("(%g).Cmp(%g) = %v; want %v", x, y, got, want)
 				}
 			}
 		}