RE: Improve CRC32C performance on SSE4.2

> Well, I suspect the AVX-512 version will pretty much always need the runtime
> check given that its not available on a lot of newer hardware and requires a
> bunch of extra runtime checks (see pg_popcount_avx512.c).  But it might be
> worth doing for PCLMUL.  Otherwise, I think we'd have to leave out the PCLMUL
> optimizations if built with -msse4.2 -mpclmul because we don't want to regress
> existing -msse4.2 users with a runtime check.

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.

Raghuveer