[PATCH 4/4] write_mem_fence() support for R600 targets

This patch introduces three new intrinsics and therefore
  must be used in conjunction with the patches to the LLVM backend. It adds the
  write_mem_fence built-in to libclc, and an implementation for r600 using the
  newly introduced LLVM intrinsics. Plain copy&paste from the read_mem_fence
  patch, with some find&replace.

Signed-off-by: Damien Hilloulin <damien.hilloulin@supelec.fr>

This patch introduces three new intrinsics and therefore
must be used in conjunction with the patches to the LLVM backend.
It adds the
write_mem_fence built-in to libclc, and an implementation for r600
using the
newly introduced LLVM intrinsics. Plain copy&paste from the read_mem_fence
patch, with some find&replace.

Same comment as patches 2 and 3.