Gdb misdetects threads (SA_ONSTACK?)

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

On a ~month old current, GNU gdb (GDB) 11.0.50.20200914-git seems to
get very confused.  My uninformed guess is it's alternative stacks for
signal handlers that confuse it and make it think a program is
threaded when it's not.  E.g.

$ gdb -q --args ./Self ...
Reading symbols from ./Self...
(No debugging symbols found in ./Self)
(gdb) r
...
^C
Thread 2 received signal SIGINT, Interrupt.
[Switching to process 1470]
0x0818be0e in zone::flushZombies() ()
(gdb) info threads
  Id   Target Id                   Frame 
  1    LWP 1470 of process 1470 "" 0x0818be0e in zone::flushZombies() ()
* 2    process 1470                0x0818be0e in zone::flushZombies() ()
(gdb) display/i $eip
1: x/i $eip
=> 0x818be0e <_ZN4zone12flushZombiesEv+86>:	mov    %eax,-0x20(%ebp)
(gdb) si
[New process 1470]
/home/uwe/work/netbsd/ro/src/external/gpl3/gdb/lib/libgdb/../../dist/gdb/infrun.c:5683: internal-error: int finish_step_over(execution_control_state*): Assertion `ecs->event_thread->control.trap_expected' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) 
(gdb) display/i $eip
1: x/i $eip
=> 0x818be0e <_ZN4zone12flushZombiesEv+86>:	mov    %eax,-0x20(%ebp)
(gdb) si
[New process 1470]
/home/uwe/work/netbsd/ro/src/external/gpl3/gdb/lib/libgdb/../../dist/gdb/infrun.c:5683: internal-error: int finish_step_over(execution_control_state*): Assertion `ecs->event_thread->control.trap_expected' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) 

-uwe