Fix code for probing availability of AVX-512.

Fix code for probing availability of AVX-512.

This commit fixes a few things:
* Instead of checking for CPU support of the "xsave" extension, we
  need to check for OS support of XGETBV instructions via the
  "osxsave" flag.
* We must check that additional XCR0 bits are set to be sure the
  ZMM registers are fully enabled.
* We should use the recommended ordering of steps.  Specifically,
  we need to check that the ZMM registers are enabled prior to
  checking for AVX-512 via CPUID.

In passing, split this code into separate functions to improve
readability.

Reported-by: Andrew Kane
Reviewed-by: Akash Shankaran, Raghuveer Devulapalli
Discussion: https://postgr.es/m/20240418024459.GA3385227%40nathanxps13