Set linkonce ODR linkage for forwarding methods
Bugfix: permit type assertion at package scope
Remove llgo-link tool

Will resurrect if needed
Tidy up llgo-dist

- No longer "generating" syscall/math packages. Instead we
  overlay all files from the llgo/pkg/<package> directory,
  and require a .ll file for each .s file found in the
  standard package.

- Added dummy .ll files for math, sync/atomic. Later the
  math ones may be filled in.

- Added compilation of C sources, using clang.