port-arm/58572: aarch64 RNDRRS failures should be evcounted and dtraced

[campbell+netbsd%mumble.net@localhost]

>Number:         58572
>Category:       port-arm
>Synopsis:       aarch64 RNDRRS failures should be evcounted and dtraced
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    port-arm-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Aug 10 14:15:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10
>Organization:
The NetRNDRRS Failedation
>Environment:
>Description:
Booting on a machine with RNDRRS (Oracle Cloud Infrastructure VM.Standard.A2.Flex instance):

[     3.692453] entropy: best effort
[     5.693122] entropy: ready

In order to confirm that this delay was due to hardware limitations, and otherwise do science on it, we need to be able to count (and ideally dtrace) when the RNDRRS instruction has failed.
>How-To-Repeat:
boot NetBSD on an OCI A2 instance
>Fix:
Yes, please!