Add platform-specific flags for NEON.

[QuLogic]

Along with some upstream changes (below), this allows a full build on armv7hl.

Auburn/FastNoiseSIMD@3287340
Auburn/FastNoiseSIMD@575c004
Auburn/FastNoiseSIMD#27
Auburn/FastNoiseSIMD#32

[QuLogic]

Ah, I wasn't too sure how well this would work on Windows; will have to tweak it a little bit.

[QuLogic]

OK, fixed Windows (I think) and also aarch64, which supports NEON, but doesn't need a flag for it.

[robbmcleod]

I think then that I should investigate making the pyfastnoisesimd\fastnoisesimd directory a submodule. I think the only added file is x86_flags.h, which doesn't need to be in the repo.

Edit: x86_flags.h is being defined to replace some macros in FastNoiseSIMD.h which otherwise require the user to commit out/in lines before compilation. So (speaking to myself here), what are the options here? Do a PR against FastNoiseSIMD to make a separate defines file? Some string-replacements in the Python setup.py?

[QuLogic]

PS, I've heard through some NumPy issues that Shippable is starting to provide ARMv8 CI.

[robbmcleod]

Good to know thanks. I see Auburns still hasn't accepted your PR so I could rebase off your repo for now and forget the submodule in the short-term.