For some reason the absense of an implementation does not stop arm64
binaries being built. However it comes up with -buildmode=c-archive.

Change-Id: Ic0db5fd8fb4fe8252b5aa320818df0c7aec3db8f
Reviewed-on: https://go-review.googlesource.com/8989Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>