port-sparc/54718: sparc install hangs since recent scheduler changes

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

>Number:         54718
>Category:       port-sparc
>Synopsis:       sparc install hangs since recent scheduler changes
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-sparc-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Nov 25 10:55:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2019.11.23.22.35.08
>Organization:

>Environment:
System: NetBSD
Architecture: sparc
Machine: sparc
>Description:

Installing NetBSD-current/sparc under qemu, everything goes fine until
sysinst asks:

  On which disk do you want to install NetBSD?

and you type "a <enter>" to choose the first available disk.  Then,
instead of sysinst continuing to the next screen, nothing happens.

The problem appeared with these commits:

  2019.11.23.22.35.08 ad src/sys/kern/kern_runq.c 1.49
  2019.11.23.22.35.08 ad src/sys/kern/sched_4bsd.c 1.37

Logs at:

  http://releng.netbsd.org/b5reports/sparc/commits-2019.11.html#2019.11.23.22.35.08

Found using bracket and anita, verified by running qemu manually, not
tested on real hardware.

>How-To-Repeat:

>Fix: