Install XLA as single shared library for each architecture

[joelberkeley]

For each architecture (CPU, GPU, TPU), we want users to only need to install one shared XLA library. Having just one shared library will make installation conceptually simpler, with fewer installation artifacts and perhaps fewer steps too.

We can do this by replacing elixir-nx's dynamic library with our own static library. This will need a new repo to replace elixir-nx's XLA repo. When we build the C wrapper against the resulting static library in the spidr repo, we'll then have a single shared library.

[joelberkeley]

I'm not sure it's worth simplifying the installation process (especially once we make the instructions run as part of postinstall). However, it would be nice to decouple from elixir-nx and build directly from the new openxla project