Hide avx512 function definitions from headers

[lemire]

The very latest version of Visual Studio appears to turn pure AVX code into AVX-512 binaries if the function, at any point could run AVX-512 instructions. This cause a regression for us.

We have code that looks like this...

int bitset_container_compute_cardinality(const bitset_container_t *bitset) {
    int support = croaring_hardware_support();
#if CROARING_COMPILER_SUPPORTS_AVX512
    if( support & ROARING_SUPPORTS_AVX512 ) {
      return (int) avx512_vpopcount(
        (const __m512i *)bitset->words,
        BITSET_CONTAINER_SIZE_IN_WORDS / (WORDS_IN_AVX512_REG));
    } else
#endif // CROARING_COMPILER_SUPPORTS_AVX512
    if( support & ROARING_SUPPORTS_AVX2 ) {
      return (int) avx2_harley_seal_popcount256(
        (const __m256i *)bitset->words,
        BITSET_CONTAINER_SIZE_IN_WORDS / (WORDS_IN_AVX2_REG));
    } else {
      return _scalar_bitset_container_compute_cardinality(bitset);

    }
}

This is likely fine as long as avx512_vpopcount does not get inlined into bitset_container_compute_cardinality. But clearly, if avx512_vpopcount gets inline, it makes it likely that Visual Studio might decide to just go AVX-512 all in.

This PR makes avx512_vpopcount and related functions regular (non-inline) functions. Hopefully, this is sufficient.

Reference: RoaringBitmap/croaring-rs#128

[Dr-Emann]

Suggesting we close this, there doesn't appear to be any amount of hiding the AVX512 code we can do to fix it, because the bug doesn't require any AVX512 source code at all: RoaringBitmap/croaring-rs#128 (comment)