gallivm: Prefer the standard JIT engine whenever possible.

gallivm: Prefer the standard JIT engine whenever possible.

Testing shows that the standard JIT engine retrofited with AVX support is quite
stable and as capable to handle AVX instructions as MC-JIT is.

And the old JIT is much more memory efficient, as we don't need to
allocate one engine instance per shader, as we do for MC-JIT due to its
incompleteness.

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