libstdc++: Fix typos in tests using macros for std::float128_t support

libstdc++: Fix typos in tests using macros for std::float128_t support

These tests check `_GLIBCXX_DOUBLE_IS_IEEE_BINARY128` but that's never
defined, it should be "LDOUBLE" not "DOUBLE".

libstdc++-v3/ChangeLog:

	* testsuite/26_numerics/complex/ext_c++23.cc: Fix typo in macro.
	* testsuite/26_numerics/headers/cmath/constexpr_std_c++23.cc:
	Likewise.
	* testsuite/26_numerics/headers/cmath/functions_std_c++23.cc:
	Likewise.
	* testsuite/26_numerics/headers/cmath/nextafter_c++23.cc:
	Likewise.