Refactor some SIMD and popcount macros.

Refactor some SIMD and popcount macros.

This commit does the following:

* Removes TRY_POPCNT_X86_64.  We now assume that the required CPUID
intrinsics are available when HAVE_X86_64_POPCNTQ is defined, as we
have done since v16 for meson builds when
USE_SSE42_CRC32C_WITH_RUNTIME_CHECK is defined and since v17 when
USE_AVX512_POPCNT_WITH_RUNTIME_CHECK is defined.

* Moves the MSVC check for HAVE_X86_64_POPCNTQ to configure-time.
This way, we set it for all relevant platforms in one place.

* Moves the #defines for USE_SSE2 and USE_NEON to c.h so that they
can be used elsewhere without including simd.h.  Consequently, we
can remove the POPCNT_AARCH64 macro.

* Moves the #includes for pg_bitutils.h to below the system headers
in pg_popcount_{aarch64,x86}.c, since we no longer depend on macros
from pg_bitutils.h to decide which system headers to use.

Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/aWf_InS1VrbeXAfP%40nathan