gallivm: use llvm jit code for decoding s3tc

gallivm: use llvm jit code for decoding s3tc

This is (much) faster than using the util fallback.
(Note that there's two methods here, one would use a cache, similar to
the existing code (although the cache was disabled), except the block
decode is done with jit code, the other directly decodes the required
pixels. For now don't use the cache (being direct-mapped is suboptimal,
but it's difficult to come up with something better which doesn't have
too much overhead.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>