Enable prefetch on MSVC

[sschoener]

Enables support for prefetch intrinsics on MSVC.

[ivmai]

we have shipped without any of this for the longest time

@sschoener, would be good to have some benchmarking about the proposed change.

hence we should exclude ARM, i.e. wrap in !defined(_M_ARM) && !defined(_M_ARM64)

It would be better to write the conditions in the opposite way, e.g.: (_M_IX86 >= 300) || defined(_M_X64)

[ivmai]

Upstreamed with some minor modifications: https://github.com/ivmai/bdwgc/commit/703fde0aec3141540745711adef0538c79f69420