cmd/internal/ld: use a simpler cout writer

Removes the unused *bufio.Reader from the object controlling the
linker's primary output.

Change-Id: If91d9f60752f3dc4b280f35d6eb441f3c47574b2
Reviewed-on: https://go-review.googlesource.com/9362


Reviewed-by: Ian Lance Taylor <iant@golang.org>