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-7.0.0.src/cmake/modules/CheckAtomic.cmake

===================================================================

--- llvm-7.0.0.src.orig/cmake/modules/CheckAtomic.cmake

+++ llvm-7.0.0.src/cmake/modules/CheckAtomic.cmake

@@ -26,8 +26,10 @@ 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);

   return 0;

 }

 " ${varname})

​