ART: Allow unwinding unattached threads

Partial revert of commit ed8b723c5f3989d2593ec21c65c96d6d8bf25579.

Make it (constexpr) configurable whether we allow unwinding native
stacks of unattached threads.

Change-Id: Ibb1fd0956fb796583f78ec9c3a74521f6fbe96d9
1 file changed