RE: Improve CRC32C performance on SSE4.2

> > Sounds good to me. Although, users building with just -msse4.2 will
> > now encounter an an additional pclmul runtime check. That would be a
> > regression unless they update to building with both -msse4.2 and -mpclmul.
>
> My thinking was that building with just -msse4.2 would cause the existing SSE 4.2
> implementation to be used (without the function pointer).  That's admittedly a bit
> goofy because they'd miss out on the PCLMUL optimization, but things at least
> don't get any worse for them.

Right. We are only talking about a regression for small potion of people who build
with -msse4.2 and run on Nehalem/VM with pclmul disabled where we will run
the cpuid check for pclmul and still pick the sse4.2 version.

Raghuveer