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