Fix check for thread unwind.

If a process requires executing fallback unwinder and the thread
crashing is not the main thread, the wrong unwinder is used.
Fix this case, and add a new unit test that causes an abort in
the non main thread.

Bug: 233721755

Test: New unit test passes with fix and fails without.
Test: Ran debuggerd on swcodec process and it still dumps all threads.
Change-Id: I70fffc5d680256ce867e7a1d427593b584259160