[compiler-rt] [TSan] Fix spurious 'thread finished with ignores enabled' warning on FreeBSD (PR #155399)

[Dan Blackwell via llvm-commits]

DanBlackwell wrote:

I tried writing a test for this; using `thr_create` to create the thread, `thr_exit` to exit, and `thr_kill(tid, 0)` to detect the exit from the main thread seems to flakily pass - with failures producing a long stream of `ThreadSanitizer: DEADLYSIGNAL`. There is not currently have an interceptor for `thr_create` currently, which may be related.

https://github.com/llvm/llvm-project/pull/155399