[dev.link] cmd/link: support new object file format
Parse new object file format in the linker. At least we can link
a hello-world program.
Add a basic "loader", which handles symbol references in the
object file.
- mapping between local and global indices
- resolve by-name references
(TODO: the overwrite logic isn't implemented yet)
Currently we still create sym.Symbol rather early, and, after all
the object files are loaded and indexed references are resolved,
add all symbols to sym.Symbols.
The code here is probably not going in the final version. This
is basically only for debugging purposes -- to make sure the
writer and the reader work as expected.