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.

libstdc++-v3/include/bits/basic_string.h

@@ -36,7 +36,6 @@
 
 #pragma GCC system_header
 
-#include <ext/atomicity.h>
 #include <ext/alloc_traits.h>
 #include <debug/debug.h>
 

libstdc++-v3/include/bits/cow_string.h

@@ -34,6 +34,8 @@
 
 #if ! _GLIBCXX_USE_CXX11_ABI
 
+#include <ext/atomicity.h> // _Atomic_word, __is_single_threaded
+
 #ifdef __cpp_lib_is_constant_evaluated
 // Support P1032R1 in C++20 (but not P0980R1 for COW strings).
 # define __cpp_lib_constexpr_string 201811L