a couple of options for testing builtin functions

--builtins | --no-builtins: enable | disable to generate builtin functions (disabled by default)
--builtin-function-prob <num>: set the probability of choosing a builtin function (default is 20)
added support for builtin functions
get the corresponding type based on a string

supported type: Void, Char, UChar, Short, UShort, Int,
                 UInt, Long, ULong, Longlong, ULonglong
done with builtin support

* Csmith remains the same behavior unless --builtins is passed
* only void and int-family types are allowed for a builtin's
  signature