RE: Improve CRC32C performance on SSE4.2

> Here's another idea to make it more automatic: Give up on initializing every
> capability at once. 

I'm not sure I like giving up this. Initializing and running CPUID check with the attribute constructor is very
valuablefor two reasons: (1) you get everything done at load time before main and (2) you don’t have to run cpuid check
forevery feature (popcount, crc32c, or anything else you add in the future) multiple times. It keep the cpuid
functionalityin a central place that makes it a modular design. 
 

On MSVC, we could have the first SIMD feature call pg_cpucap_initialize() which runs CPUID stores the cpu features. Any
subsequentcall can skip (because it has already been initialized) by using a static variable or some other approach.
Doesthis make sense? 
 

Raghuveer