OpenMP: No memset in viewfill (kokkos#6573)

* OpenMP: Edit copyview implementation.

* OpenMP: Use memset for Serial backend.

* Update core/src/Kokkos_CopyViews.hpp

OpenMP: do not use memset for 0's only if execution space is OpenMP

Co-authored-by: Daniel Arndt <arndtd@ornl.gov>

* Fix incorrect code.

---------

Co-authored-by: Rahulkumar Gayatri <rgayatri@lbl.gov>
Co-authored-by: Daniel Arndt <arndtd@ornl.gov>

core/src/Kokkos_CopyViews.hpp

@@ -1348,13 +1348,14 @@ inline std::enable_if_t<
 contiguous_fill_or_memset(
     const ExecutionSpace& exec_space, const View<DT, DP...>& dst,
     typename ViewTraits<DT, DP...>::const_value_type& value) {
-// On A64FX memset seems to do the wrong thing with regards to first touch
-// leading to the significant performance issues
-#ifndef KOKKOS_ARCH_A64FX
-  if (Impl::is_zero_byte(value))
+  // With OpenMP, using memset has significant performance issues.
+  if (Impl::is_zero_byte(value)
+#ifdef KOKKOS_ENABLE_OPENMP
+      && !std::is_same_v<ExecutionSpace, Kokkos::OpenMP>
+#endif
+  )
     ZeroMemset<ExecutionSpace, View<DT, DP...>>(exec_space, dst, value);
   else
-#endif
     contiguous_fill(exec_space, dst, value);
 }