The std-clib.cc module definition file assumes that all names are
available unconditionally, but that's not true for all targets. Use the
same preprocessor conditions as are present in the <cxxx> headers.

A similar change is needed in std.cc.in for the <chrono> features that
depend on the SSO std::string, guarded with a __cpp_lib_chrono value
indicating full C++20 support.

The conditions for <cmath> are omitted from this change, as there are a
large number of them. That probably needs to be fixed.

libstdc++-v3/ChangeLog:

	PR libstdc++/118177
	* src/c++23/std-clib.cc.in: Use preprocessor conditions for
	names which are not always defined.
	* src/c++23/std.cc.in: Likewise.