runtime: expose auxv for use by x/sys/cpu

runtime: expose auxv for use by x/sys/cpu

Updates #57336

Change-Id: I181885f59bac59360b855d3990326ea2b268bd28
Reviewed-on: https://go-review.googlesource.com/c/go/+/458256
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>