pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/598e0114a3b16bb7edcaba6784b3794f816889ae

Modified Files
--------------
src/port/pg_popcount_avx512_choose.c | 80 +++++++++++++++++++++---------------
1 file changed, 47 insertions(+), 33 deletions(-)