Starting with Xcode 12 Beta 2, builds that used to work on Apple Silicon
now break due to `Hardware#oldest_cpu` returning `:nehalem` [1].
This commit is the first in a series of improvements to
`Hardware#oldest_cpu`. It resolves the Xcode 12 Beta 2 issue for now.
[1]: https://github.com/Homebrew/brew/issues/7857#issuecomment-655536261