[dev.link] cmd/link: stream external relocations on ARM64 and on Darwin

[dev.link] cmd/link: stream external relocations on ARM64 and on Darwin

Support streaming external relocations on ARM64. Support
architecture-specific relocations.

Also support streaming external relocations on Darwin. Do it in
the same CL so ARM64's archreloc doesn't need to support both
streaming and non-streaming.

Change-Id: Ia7fee9957892f98c065022c69a51f47402f4d6e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/243644
Reviewed-by: Jeremy Faller <jeremy@golang.org>