port-macppc/55793: NetBSD/macppc fails to boot since switch to gcc9

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

>Number:         55793
>Category:       port-macppc
>Synopsis:       NetBSD/macppc fails to boot since switch to gcc9
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    port-macppc-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Nov 07 15:25:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2020.10.09.05.41.16
>Organization:
>Environment:
System: NetBSD
Architecture: powerpc
Machine: macppc
>Description:

Testing NetBSD-current/macppc under qemu, installation succeeds, but
booting the installed system fails with:

  [   1.0000000] panic: lock error: Mutex: mutex_vector_enter,543: locking against myself: lock 0xbf9740 cpu 0 lwp 0xbf3d40

since this commit:

  2020.10.09.05.41.16 mrg src/share/mk/bsd.own.mk 1.1224

  switch powerpc, sh3, vax and m68k to GCC 9.  that's everyone.

  thanks espcially to rin@ for testing and fixing issues.

Logs:

  https://www.gson.org/netbsd/bugs/build/macppc/commits-2020.10.html#2020.10.09.05.41.16

>How-To-Repeat:

>Fix: