libstdc++: Only include <ext/atomicity.h> for COW string

libstdc++: Only include <ext/atomicity.h> for COW string

Since the COW std::string was moved to its own header, we don't need the
atomic dispatch helpers in the definition of std::__cxx11::string. Move
the inclusion of the <ext/atomicity.h> header to <bits/cow_string.h>
where it's needed.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h: Do not include <ext/atomicity.h>
	here.
	* include/bits/cow_string.h: Include it here.