[AMDGPU][PATCH 3/3] Stubs implementation of the new intrinsics on Southern_Islands

This patch is a first implementation of the newly added
intrinsics for barriers/memory fences on SI. For ultra-simplicity, every
intrinsic is lowered to a barrier with no fence.

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