Revert "[dev.link] cmd/link: remove buffered file I/O from OutBuf"

This reverts commit b2def42d.

Reason for revert: trybots failing

Change-Id: I920be6d8de158b1e513154ac0eb0c8fa0cffa9f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/228657


Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>