Spotted while trying to link a program using unix.Syscall, since its
implementation is assembly.
Telling if a function couldn't be garbled isn't trivial. If that
function belongs to an imported package, we only load its export data
instead of type-checking from source, so we don't have all the
information needed.
Instead, use the gc export data importer to import two versions of each
dependency: its original version, for the initial type-checking, and its
garbled version, to check if any of its exported names weren't garbled.
Updates #9.
Daniel Martí