Add windows/arm64 support, ensure unsupported platforms see a compile error

[cesaref]

I've added some updates to support building on windows/arm64.

The tests were previously failing as it was passing through an empty implementation for the memcpy functions, so i've altered these to give a compile error if this happens.

On windows with C++20 enabled, the tests pass on both x64 and arm64 (going via the __cpp_lib_atomic_ref codepath)

[triplef]

While we’re at it would be great to add support for Android with __aarch64__ as well (see docs).

Suggested change

	#elif defined (__arm64__) || defined (_M_ARM64)
	#elif defined (__arm64__) || defined (__aarch64__) || defined (_M_ARM64)

[timuraudio]

Hi @cesaref I checked out your PR today and it's good stuff, I would like to merge it but GitHub says "This branch cannot be rebased due to conflicts". I did resolve the conflict that GitHub flagged, which resulted in a commit on your branch, but somehow GitHub still thinks there's a conflict. Can you resolve it? Once you do I'm happy to merge this PR as-is. Thanks!

[timuraudio]

Alright, I had another look — I have no idea why GitHub doesn't want to automatically merge this PR, all changes seem straightforward. I just applied them manually instead so I'm closing this PR. Thanks again for contributing these fixes!!