port-sparc64/54719: sparc64 fails to boot since switch to gcc8

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

>Number:         54719
>Category:       port-sparc64
>Synopsis:       sparc64 fails to boot since switch to gcc8
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-sparc64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Nov 25 14:10:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2019.11.16.10.23.36
>Organization:
>Environment:
System: NetBSD
Architecture: sparc64
Machine: sparc64
>Description:

The TNF sparc64 testbed is failing to boot from the installation media
since this commit:

  2019.11.16.10.23.36 mrg src/share/mk/bsd.own.mk 1.1162

with the commit message

  sparc & sparc64 -> GCC 8.
  
The console log messages look like this:

  Welcome to OpenBIOS v1.1 built on Jul 1 2019 17:08
    Type 'help' for detailed information
  Trying cdrom:f...
  Not a bootable ELF image
  Not a bootable a.out image

  Loading FCode image...
  Loaded 7514 bytes
  entry point is 0x4000
  Evaluating FCode...
  NetBSD IEEE 1275 Multi-FS Bootblock
  Version $NetBSD: bootblk.fth,v 1.15 2015/08/20 05:40:08 dholland Exp $
  ..Unhandled Exception 0x0000000000000030
  PC = 0x00000000ffd0f2b8 NPC = 0x00000000ffd0f2bc
  Stopping execution

This does not affect sparc, only sparc64.

>How-To-Repeat:

>Fix: