libstdc++: Use _GLIBCXX_USE_BUILTIN_TRAIT for _Nth_type

Since _Nth_type has a fallback native implementation, use
_GLIBCXX_USE_BUILTIN_TRAIT when checking for __type_pack_element
so that we can easily toggle which implementation to use.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (_Nth_type): Use
	_GLIBCXX_USE_BUILTIN_TRAIT instead of __has_builtin.