`rustc` allows passing in predefined target triples as well as JSON
target specification files. This change allows bootstrap to have the
first inkling about those differences. This allows building a
cross-compiler for an out-of-tree architecture (even though that
compiler won't work for other reasons).

Even if no one ever uses this functionality, I think the newtype
around the `Interned<String>` improves the readability of the code.