port-amd64/54775: PXE netboot fails again

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

>Number:         54775
>Category:       port-amd64
>Synopsis:       PXE netboot fails again
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Dec 17 13:50:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2019.12.15.02.56.40
>Organization:
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Netbooting NetBSD-current/amd64 using PXE is failing since these
commits:

   2019.12.15.02.56.40 manu src/sys/arch/amd64/amd64/locore.S 1.194
   2019.12.15.02.56.40 manu src/sys/arch/amd64/conf/kern.ldscript 1.30

Compared to PR 54560 which was fixed in September, pxeboot now gets a
bit further before restarting:

   Server powering on .......
   connected
    5nX
   >> NetBSD/x86 PXE boot, Revision 5.1 (Sun Dec 15 02:56:40 UTC 2019) (from NetBSD 9.99.24)
   >> Memory: 552/3668992 k
   Press return to boot now, any other key for boot menu
   booting netbsd - starting in 0 seconds.
   PXE BIOS Version 2.1
   Using PCI device at bus 3 device 0 function 0
   Ethernet address 98:4b:e1:67:68:98
   0+15651440/ 5nX
   >> NetBSD/x86 PXE boot, Revision 5.1 (Sun Dec 15 02:56:40 UTC 2019) (from NetBSD 9.99.24)
   >> Memory: 552/3668992 k
   Press return to boot now, any other key for boot menu
   booting netbsd - starting in 0 seconds.
   PXE BIOS Version 2.1
   Using PCI device at bus 3 device 0 function 0
   Ethernet address 98:4b:e1:67:68:98
   0+15651440/ 5nX
   >> NetBSD/x86 PXE boot, Revision 5.1 (Sun Dec 15 02:56:40 UTC 2019) (from NetBSD 9.99.24)
   >> Memory: 552/3668992 k
   Press return to boot now, any other key for boot menu
   booting netbsd - starting in 0 seconds.

For more log output, see:

   http://www.gson.org/netbsd/bugs/build/amd64-baremetal/commits-2019.12.html#2019.12.15.02.56.40

>How-To-Repeat:

Attempt to netboot NetBSD-current/amd64 using PXE.

>Fix: