cmd/internal/ld: delete Biobuf

Update #10652

This proposal deletes cmd/internal/ld.Biobuf and replaces all uses with
cmd/internal/obj.Biobuf. As cmd/internal/ld already imported cmd/internal/obj
there are no additional dependencies created.

Notes:

- ld.Boffset included more checks, so it was merged into obj.Boffset
- obj.Bflush was removed in 8d16253c, so replaced all calls to
  ld.Bflush, with obj.Biobuf.Flush.
- Almost all of this change was prepared with sed.

Change-Id: I814854d52f5729a5a40c523c8188e465246b88da
Reviewed-on: https://go-review.googlesource.com/9660


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>