swr/rast: ignore CreateElementUnorderedAtomicMemCpy

This function's API changed between LLVM 5 and 6. Compile errors occur
when building with LLVM 6+ if LLVM 5 was used for a dist tarball

CC: <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107865


Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit edf38019)