gh-109054: Don't use libatomic on cross-compilation

configure no longer uses libatomic by default when Python is
cross-compiled. The LIBATOMIC variable can be set manually in this
case:

    ./configure LIBATOMIC="-latomic" (...)

Author: vstinner

configure

@@ -7007,9 +7007,10 @@ int main()
     }
     return 0; // all good
 }
-]])],[
-  ac_cv_libatomic_needed=no
-],[ac_cv_libatomic_needed=yes],[ac_cv_libatomic_needed=yes])
+]])],
+[ac_cv_libatomic_needed=no],  dnl build succeeded
+[ac_cv_libatomic_needed=yes], dnl build failed
+[ac_cv_libatomic_needed=no])  dnl cross compilatio
 ])
 
 AS_VAR_IF([ac_cv_libatomic_needed], [yes],