bugfix: fix spack buildcache list --allarch

`spack buildcache list` was trying to construct an `Arch` object and
compare it to `arch_for_spec(<spec>)`. for each spec in the buildcache.
`Arch` objects are only intended to be constructed for the machine they
describe. The `ArchSpec` object (part of the `Spec`) is the descriptor
that lets us talk about architectures anywhere.

- [x] Modify `spack buildcache list` and `spack buildcache install` to
      filter with `Spec` matching instead of using `Arch`.