Re: Popcount optimization using AVX512

It was brought to my attention [0] that we probably should be checking for
the OSXSAVE bit instead of the XSAVE bit when determining whether there's
support for the XGETBV instruction.  IIUC that should indicate that both
the OS and the processor have XGETBV support (not just the processor).
I've attached a one-line patch to fix this.

[0] https://github.com/pgvector/pgvector/pull/519#issuecomment-2062804463

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com