add cpu_features

[Crghilardi]

This PR adds a build_tarballs.jl for the cpu_features library.

This works for me on the regular platforms I test on, but there are a few commits on master that are not in the release that I may need to pull in still for the apple-* platforms.

The top level CMake file also strongly recommends building as a static library, so I didn't mess with anything and just produce the executable currently.

[vchuravy]

How do you plan to use it? From Julia you will need a shared library, or a costly shell out to the command line.

Do you need it as a dependency for something else?

[Crghilardi]

I was hoping to use it as a dependency for volk. They have cpu_features as a submodule, but I thought bringing it in as a JLL and modifying the cmake to point to the JLL instead might be better?

I need to do some testing, but looking at it now I think I should be able to apply the patch from here to the submodule instead and then we can close this.

[giordano]

Since the only point of this was to be a dependency of VOLK and VOLK has been built in #3718 using cpu_features as a static library (?), maybe this can be closed?