libstdc++: Improve 26_numerics/headers/cmath/types_std_c++0x_neg.cc

libstdc++: Improve 26_numerics/headers/cmath/types_std_c++0x_neg.cc

This test checks that the special functions in <cmath> are not declared
prior to C++17. But we can remove the target selector and allow it to be
tested for C++17 and later, and add target selectors to the individual
dg-error directives instead.

Also rename the test to match what it actually tests.

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/headers/cmath/types_std_c++0x_neg.cc:
	Move to ...
	* testsuite/26_numerics/headers/cmath/specfun_c++17.cc: here and
	adjust test to be valid for all -std dialects.