Add FeatureFlatForGlobal to Volcanic Islands features and update the tests to contain -mattr=-flat-for-global.
This patch is intended for 4.0 branch to avoid assert failure on Volcanic Islands due to missing ADDR64 instructions. There is a better approach targeted for trunk based on arsenm's patches: https://github.com/arsenm/llvm/tree/movetovalu-addr64 which I will post at a later date.