Split auxv header into its own module

[morr0ne]

As the title says. This PR splits up the auxv header into its own module. This seems trivial but it has ended up helping me a lot in some projects for several reasons.

• Having a clear separation of the module instead of the the constants living in the general module allows me to more clearly see what's actually part of the auxv. Unlike other definitions, the auxv don't have any kind of differentiating prefix but just start with AT_ which is also used by other consts. A good way to illustrate the point is the counter example of the magic.h consts which all have the MAGIC suffix so they are easy to differentiate
• Makes it easier to differentiate between architectures since not all architectures have the same auxv, so it makes it relatively trivial to compare them
• Overall it's a much nicer way of accessing these variables since it clearly denotes where they are coming from and I can use them with a qualified auxv::AT_ import

Implementing this made me realize that there are other modules bunched up in the general.h that could ideally be separate modules, but it would get really crowded if all of those modules had their own feature. Not sure if it's worth opening an issue to discuss that.