IB/core: Add ib headers for general use

Add IB headers, defines, and accessors that are identical
in both qib and hfi1 into the core includes.

The accessors for be maintenance of __be64 fields since
alignment is potentially invalid and can differ based on
the presense of the GRH.

{hfi1,qib}_ib_headers will be ib_headers.
{hfi1,qib|_other_headers will be ib_other_headers.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Don Hiatt <don.hiatt@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>