Race on lock owner

Didn't look at this much, but

```
WARNING: ThreadSanitizer: data race (pid=60527)
Read of size 8 at 0x000100121568 by thread T13 (mutexes: write M991771789040377880, write M965876331701207728):
#0 deliver_signal_unlocked (ish:x86_64+0x10001c7f6)
#1 send_signal (ish:x86_64+0x10001d084)
#2 itimer_notify (ish:x86_64+0x100016c92)
#3 timer_thread (ish:x86_64+0x100067729)

Previous write of size 8 at 0x000100121568 by thread T15 (mutexes: write M3840):
#0 unlock (ish:x86_64+0x100019c9a)
#1 futex_put (ish:x86_64+0x1000193ea)
#2 futex_wake (ish:x86_64+0x1000194c5)
#3 sys_futex (ish:x86_64+0x100019661)
#4 handle_interrupt (ish:x86_64+0x100004976)
#5 task_run_current (ish:x86_64+0x100007705)
#6 task_thread (ish:x86_64+0x100007821)
```

```console
(lldb) image lookup -a 0x10001c7f6
Address: ish[0x000000010001c7f6] (ish.__TEXT.__text + 109446)
Summary: ish`deliver_signal_unlocked + 774 at signal.c:78:55
(lldb) image lookup -a 0x100019c9a
Address: ish[0x0000000100019c9a] (ish.__TEXT.__text + 98346)
Summary: ish`unlock + 442 at sync.h:62:17
(lldb) image lookup -a 0x1000193ea
Address: ish[0x00000001000193ea] (ish.__TEXT.__text + 96122)
Summary: ish`futex_put + 122 at futex.c:54:5
```

Seems like there is a race on the lock's owner in `deliver_signal_unlocked` when the `trylock` fails but we check it anyways.

Start with the ThreadSanitizer trace in signal.c:78, then inspect the lock handling in sync.h:62 and futex.c:54. Reproduce the race under ThreadSanitizer and trace how deliver_signal_unlocked observes the lock owner after trylock fails. Done means the race is resolved and the sanitizer no longer reports it.

Written by the indexing model from the issue text.