RE: Improve CRC32C performance on SSE4.2

Hello John, 

v15 LGTM. Couple of minor comments: 
 
> I'm leaning towards a length limit for v15-0001 so that inlined instructions are
> likely to be unrolled. Aside from lack of commit message, I think that one is ready
> for commit soon-ish.

+1 

> I'm feeling pretty good about 0002, but since there is still room for cosmetic
> fiddling, I want to let it sit for a bit longer.

(1) zmm_regs_available() in pg_crc32c_sse42_choose.c is a duplicate of pg_popcount_avx512.c but perhaps that’s fine for
now.I will propose a consolidated SIMD runtime check in a follow up patch. 
 
(2) Might be apt to rename pg_crc32c_sse42*.c to pg_crc32c_x86*.c  since they contain both sse42 and avx512 versions.

> I felt the previous proposals for configure.ac were unnecessarily invasive, and the
> message looked out of place, so I made configure.ac more similar to master, using
> the AVX popcount stuff as a model. 

Looks good to me. 

Raghuveer