port-arm/55598: ChaCha self-test sometimes fails on evbarm-earmv7hf testbed

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         55598
>Category:       port-arm
>Synopsis:       ChaCha self-test sometimes fails on evbarm-earmv7hf testbed
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    port-arm-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Aug 22 13:35:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:
>Environment:
System: NetBSD
Architecture: earmv7hf
Machine: evbarm
>Description:

The ChaCha self-test is randomly failing on the qemu-based TNF
evbarm-earmv7hf testbed, logging this message to the console:

  chacha: self-test failed: ARM NEON ChaCha

Logs:

  http://releng.netbsd.org/b5reports/evbarm-earmv7hf/2020/2020.08.04.04.22.02/test.log
  http://releng.netbsd.org/b5reports/evbarm-earmv7hf/2020/2020.08.19.22.47.09/test.log
  http://releng.netbsd.org/b5reports/evbarm-earmv7hf/2020/2020.08.20.11.09.56/test.log

>How-To-Repeat:

Boot NetBSD-current/evbarm-earmv7hf in qemu repeatedly.

>Fix: