do not import stdatomic as it doesn't work with c++ interop on windows (#830)

* [windows] do not import stdatomic on windows as it doesn't work with c++ interop enabled

Works around swiftlang/swift#75720

* remove the stdatomic include alltogether

Author: hyp

Sources/_FoundationCShims/include/_CStdlib.h

@@ -136,10 +136,6 @@
 #include <complex.h>
 #endif
 
-#if __has_include(<stdatomic.h>)
-#include <stdatomic.h>
-#endif
-
 #if __has_include(<threads.h>)
 #include <threads.h>
 #endif