backend=auto (now the default if backend is omitted) means to let the compiler
pick whatever backend it wants as the default. This is important for platforms
where we don't yet have a self-hosted backend, such as loongarch64.
Also purge a bunch of redundant target=native.
* the root struct decl name is fully qualified
this prevents error messages containing 'main.main'
* avoid declared here note when file struct is missing a member
It always points at the start of the file which might contain another
container misleading the user.
