Update features.md to reflect minimum SIMD requirements

[CKingX]

• Updated macOS minimum SIMD support from SSSE3 to SSE4.1 as macOS Sierra and later require Penryn
• Included the minimum SIMD support for most Windows targets of SSE3

[CKingX]

I don't think the tests are failing due to the change as I only updated the features page of the performance guide.

[workingjubilee]

Oh yes, several fundamental ideas that this crate hinges on got rewritten.

[workingjubilee]

@CKingX I have no intention of merging your change, sorry! This documentation should go to the rust-lang/rust repository, for the platform support pages hereish: https://github.com/rust-lang/rust/tree/master/src/doc/rustc/src

[CKingX]

So I looked into the platform support and it doesn't make sense to put it in there. I also don't think constantly updating features.md whenever a target changes makes sense. Would it make sense to create a new PR explaining that you can look up the default target features with rustc --print cfg or for other targets, rustc --print cfg --target={target}?

[workingjubilee]

Not even in the docs for the targets? https://github.com/rust-lang/rust/tree/master/src/doc/rustc/src/platform-support

And sure...?

[CKingX]

Thanks! I will look into updating those docs.

[CKingX]

It seems a lot of the docs on the targets list if things like SSE are disabled. Would it make sense updating the Windows 7 target arch to list it only supports SSE2 by default unlike the regular targets?

Also thank you for your help!