SafeStack: Fix flaky test (PR39001)
pthread_join() can return before a thread finishes exit()ing in the
kernel and a subsequent tgkill() can report the thread still alive.
Update the pthread-cleanup.c test to sleep and retry if it hits this
Thanks to Jeremy Morse for reporting.
Reviewers: jmorse, eugenis, vitalybuka
Reviewed By: jmorse, vitalybuka
Subscribers: delcypher, jfb, llvm-commits, #sanitizers, kcc
Differential Revision: https://reviews.llvm.org/D52330