It was an oversight (but as linux/arm64 doesn't support internal
linking and always use external linking with cgo, no harm is done.)

Change-Id: Ie5f2b445cb67a8e63d6b868e63379c68847554f9
Reviewed-on: https://go-review.googlesource.com/10636Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>