libstdc++: Simplify built-in detection in <utility>

libstdc++: Simplify built-in detection in <utility>

Now that GCC supports __has_builtin there is no need to test whether
it's defined, we can just use it unconditionally.

libstdc++-v3/ChangeLog:

	* include/std/utility: Use __has_builtin without checking if
	it's defined.