File llvm-better-detect-64bit-atomics-support.patch of Package llvm8

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})
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