libstdc++: Fix std::numeric_limits::lowest() test for strict modes

libstdc++: Fix std::numeric_limits::lowest() test for strict modes

This test uses std::is_integral to decide whether we are testing an
integral or floating-point type. But that fails for __int128 because
is_integral<__int128> is false in strict modes. By using
numeric_limits::is_integer instead we get the right answer for all types
that have a numeric_limits specialization.

We can also simplify the test by removing the unnecessary tag
dispatching.

libstdc++-v3/ChangeLog:

	* testsuite/18_support/numeric_limits/lowest.cc: Use
	numeric_limits<T>::is_integer instead of is_integral<T>::value.