`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.

When running a command like `DESTDIR=foo x.py install` in a completely
clean build directory, this will cause LLVM to be installed into
`DESTDIR`, which then causes the build to fail later when it attempts
to *use* those LLVM files.

This is basic, and has at least one hack around zlib that needs to be
figured out before being merged.

We should also figure if we want to have more deduplication with the
existing Darwin target.