Set GMX_SIMD to MIC for Xeon Phi

The reason GMX_SIMD wasn't set the same wasy as it is for other architectures,
was that we wanted it to also work for offload. But for offload it is easy to
overwrite it for a compilation unit being offloaded. Setting it normally for
native mode makes it more consistent and lets the user disable SIMD.

Change-Id: Id660a6560278039673ce0a4feaea3261647f40e8