Better detect 64bit atomics support.

It appears that on i586 std::atomic<uint64_t>::load is compiled into
instruction, but std::atomic<double>::load uses __atomic_load_8. This must
be detected so the build system links it to libatomic.
Index: llvm-12.0.0.src/cmake/modules/CheckAtomic.cmake
===================================================================
--- llvm-12.0.0.src.orig/cmake/modules/CheckAtomic.cmake
+++ llvm-12.0.0.src/cmake/modules/CheckAtomic.cmake
@@ -30,9 +30,12 @@ function(check_working_cxx_atomics64 var
 #include <atomic>
 #include <cstdint>
 std::atomic<uint64_t> x (0);
+std::atomic<double> y (0);
 int main() {
   uint64_t i = x.load(std::memory_order_relaxed);
+  double j = y.load(std::memory_order_relaxed);
   (void)i;
+  (void)j;
   return 0;
 }
 " ${varname})