bin/54060: usr.bin/gdb/t_regress/threads test case now fails on amd64

[gson%gson.org@localhost]

>Number:         54060
>Category:       bin
>Synopsis:       usr.bin/gdb/t_regress/threads test case now fails on amd64
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Mar 15 10:00:00 +0000 2019
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2019.03.08.23.35.01
>Organization:

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

The usr.bin/gdb/t_regress/threads test case started failing on amd64
about a week ago.  The problem started some time between source dates
2019.03.07.18.32.10 and 2019.03.08.23.35.01; a bisection is in
progress but may take a few days.

The test passes on i386.

>How-To-Repeat:

>Fix: