Add SIMD support for Intel MIC

Only single precision is supported so far.

To compile in native mode:
CFLAGS="-mmic" CXXFLAGS="-mmic" cmake

Regressiontests pass with ICC 14.

Instructions are not updated because only offload mode (#1181)
is useful to the user.

Part of #1187

Change-Id: I81a2022cfcecf634fdfaff5ce63ad82f0a5d4dee