Limit declaration of GroupVarint tables to SSE >= 2

Limit declaration of GroupVarint tables to SSE >= 2

Summary: __m128i is only available in SSE 2 and later.
We technically only use it in folly/GroupVarint.h for SSE 3+,
but that's due to various other intrinsics and we could
potentially lower the requirement in certain contexts,
so do the right thing in the generated table.
Revisit the uses in folly/GroupVarint.h later.

Reviewed By: @yfeldblum

Differential Revision: D2445798