Re: Popcount optimization using AVX512

Okay, here is a slightly different approach that I've dubbed the "maximum
assumption" approach.  In short, I wanted to see how much we could simplify
the patch by making all possibly-reasonable assumptions about the compiler
and CPU.  These include:

* If the compiler understands AVX512 intrinsics, we assume that it also
  knows about the required CPUID and XGETBV intrinsics, and we assume that
  the conditions for TRY_POPCNT_FAST are true.
* If this is x86_64, CPUID will be supported by the CPU.
* If CPUID indicates AVX512 POPCNT support, the CPU also supports XGETBV.

Do any of these assumptions seem unreasonable or unlikely to be true for
all practical purposes?  I don't mind adding back some or all of the
configure/runtime checks if they seem necessary.  I guess the real test
will be the buildfarm...

Another big change in this version is that I've moved
pg_popcount_avx512_available() to its own file so that we only compile
pg_popcount_avx512() with the special compiler flags.  This is just an
oversight in previous versions.

Finally, I've modified the build scripts so that the AVX512 popcount stuff
is conditionally built based on the configure checks for both
autoconf/meson.

-- 
Nathan Bossart
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