Comparable#clamp is not supported

To reproduce:

```ruby
10.clamp(0, 5)
'c'.clamp('a', 'b')
```

Error:

```
NoMethodError: undefined method `clamp' for 10:Integer
	from (irb):10
	from ~/.rubies/truffleruby-1.0.0-rc10/bin/irb:29:in `<main>'

NoMethodError: undefined method `clamp' for "c":String
	from (irb):11
	from ~/.rubies/truffleruby-1.0.0-rc10/bin/irb:29:in `<main>'
```