Make TR1 special function tests support C++98

Make TR1 special function tests support C++98

	* testsuite/tr1/5_numerical_facilities/special_functions/
	08_cyl_bessel_i/check_value.cc: Use __attribute__((unused)) instead
	of [[gnu::unused]].
	* testsuite/tr1/5_numerical_facilities/special_functions/
	09_cyl_bessel_j/check_value.cc: Likewise.
	* testsuite/tr1/5_numerical_facilities/special_functions/
	10_cyl_bessel_k/check_value.cc: Likewise.
	* testsuite/tr1/5_numerical_facilities/special_functions/
	11_cyl_neumann/check_value.cc: Likewise.
	* testsuite/tr1/5_numerical_facilities/special_functions/
	21_sph_bessel/check_value.cc: Likewise.
	* testsuite/tr1/5_numerical_facilities/special_functions/
	23_sph_neumann/check_value.cc: Likewise.

From-SVN: r238985