port-amd64/53045: NetBSD-current/amd64 no longer boots from qemu hard disk

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

>Number:         53045
>Category:       port-amd64
>Synopsis:       NetBSD-current/amd64 no longer boots from qemu hard disk
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Feb 21 15:30:00 +0000 2018
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2018.02.11
>Organization:

>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Since Feb 11, every amd64 test run on the TNF testbed has failed to
boot.  The install CD boots in qemu, and system installs (unless you
happen to get hit by PR 52614), but when attempting to boot the
installed system from the emulated hard disk, there is no serial
console output, not even a banner from the boot blocks:

  http://releng.netbsd.org/b5reports/amd64/2018/2018.02.18.23.51.20/test.log

On my own testbed, using qemu 2.11.1, the symptoms are slightly
different: the system ends up in a reboot loop where the MBR boot
block prints part of its banner before the system reboots:

  http://www.gson.org/netbsd/bugs/build/amd64/2018/2018.02.21.09.47.37/test.log

On physical hardware, the system boots successfully:

  http://www.gson.org/netbsd/bugs/build/amd64-baremetal/2018/2018.02.18.15.29.29/test.log

I'm still bisecting to find the exact commit when the problem appeared,
but I'm guessing it was when amd64 was switched to gcc6.

>How-To-Repeat:

>Fix: