libstdc++: Replace dg-options "-std=c++17" with dg-add-options strict_std

These tests can (and should) also be run for later standards, so replace
the { dg-options "-std=c++17" } with { dg-add-options strict_std }.

	* testsuite/tr1/5_numerical_facilities/special_functions/07_conf_hyperg/compile_cxx17.cc:
	Replace dg-options -std=c++17 with dg-add-options strict_std.
	* testsuite/tr1/5_numerical_facilities/special_functions/17_hyperg/compile_cxx17.cc:
	Likewise.