[dev.link] cmd/link: stream external relocations on ARM and on Windows

[dev.link] cmd/link: stream external relocations on ARM and on Windows

Do them in the same CL so ARM's archreloc doesn't need to support
both streaming and non-streaming.

TODO: we haven't switched to using mmap to emit external
relocations on Windows.

Change-Id: Ica2ee89c03fc74839efd6b9e26c80585fcdce45c
Reviewed-on: https://go-review.googlesource.com/c/go/+/244357
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>