bin/54801: sdiff tests fail on aarch64 since switch to GCC 8

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

>Number:         54801
>Category:       bin
>Synopsis:       sdiff tests fail on aarch64 since switch to GCC 8
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Dec 27 09:15:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2019.10.22.13.49.30
>Organization:

>Environment:
System: NetBSD
Architecture: aarch64
Machine: evbarm
>Description:

The following test cases are failing on evbarm-aarch64:

  usr.bin/sdiff/t_sdiff/dot
  usr.bin/sdiff/t_sdiff/flags
  usr.bin/sdiff/t_sdiff/tabs
  usr.bin/sdiff/t_sdiff/merge
  usr.bin/sdiff/t_sdiff/tabends
  usr.bin/sdiff/t_sdiff/stdin
  usr.bin/sdiff/t_sdiff/same
  usr.bin/sdiff/t_sdiff/short
  usr.bin/sdiff/t_sdiff/iflags
  usr.bin/sdiff/t_sdiff/oneline

since this commit:

   2019.10.22.13.49.30 christos src/share/mk/bsd.own.mk 1.1157

   move a{md,arch}64 to gcc 8

Logs:

  http://www.gson.org/netbsd/bugs/build/evbarm-aarch64/commits-2019.10.html#2019.10.22.13.49.30
  http://releng.netbsd.org/b5reports/evbarm-aarch64/commits-2019.10.html#2019.10.22.18.43.14
  
>How-To-Repeat:

>Fix: