This change expands the stdio support on baremetal to support opaque
FILE*. This builds on top of the existing baremetal embedding API; we
treat the standard FILE* streams as pointers that point to the cookie
symbols which are a part of the embedding API. This also allows users to
define their own FILE* streams, but we don't (yet) support the API that
return FILE* such as fopen or fopencookie.
dd63127b56