OpenBSD does not provide auxv, however we still need to initialise cpu.HWCap.
For now initialise it to the bare minimum, until some form of CPU capability
detection is implemented or becomes available - see issue #31746.

Updates #31656

Change-Id: I68c3c069319fe60dc873f46def2a67c9f3d937d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/174129
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>