The current implementation for arm64 only supports linux (through HWCaps
read from /proc/self/auxv) and fails to build e.g. on darwin/arm64.

Fixes golang/go#30237

Change-Id: I9f32a285760795441a829b0a81b2d5aff04e2dc4
Reviewed-on: https://go-review.googlesource.com/c/162797
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>