This is a roll forward of 2adc3bd6. It occurred to me that we will
want this code on both darwin/arm and darwin/arm64. Removing _arm from
the file name conveniently avoids #10032.

Change-Id: I3a96a3e7020907d9307af8f696e26ad55b2060f0
Reviewed-on: https://go-review.googlesource.com/6460Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>