SafeStack: Fix flaky test (PR39001)

Summary:
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
possible flake.

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

llvm-svn: 342763