Rename TRY_POPCNT_FAST to TRY_POPCNT_X86_64.

Rename TRY_POPCNT_FAST to TRY_POPCNT_X86_64.

This macro protects x86_64-specific code, and a subsequent commit
will introduce AArch64-specific versions of that code.  To prevent
confusion, let's rename it to clearly indicate that it's for
x86_64.  We should likely move this code to its own file (perhaps
merging it with the AVX-512 popcount code), but that is left as a
future exercise.

Reviewed-by: "[email protected]" <[email protected]>
Reviewed-by: John Naylor <[email protected]>
Discussion: https://postgr.es/m/010101936e4aaa70-b474ab9e-b9ce-474d-a3ba-a3dc223d295c-000000%40us-west-2.amazonses.com