We'll need to update gallivm for the interface changes in LLVM 3.6, and
the fewer the number of older LLVM versions we support the less hairy that
will be.

As consequence HAVE_AVX define can disappear.  (Note HAVE_AVX meant
whether LLVM version supports AVX or not.  Runtime support for AVX is
always checked and enforced independently.)

Verified llvmpipe builds and runs with with LLVM 3.3, 3.4, and 3.5.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>