unix: add darwin/arm64 support

Fixes golang/go#10239.

Change-Id: I3ae657b57ca7bef31207780477c44939c0bdc8b5
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/10185
Reviewed-by: Rob Pike <r@golang.org>
7 files changed