Ensure host stack trace has better chance to work when valgrind is exiting

Ensure host stack trace has better chance to work when valgrind is exiting

When investigating bug 383275, the host stacktrace was containing
only one IP. This is because the tid corresponding to the lwpid
is dead, and so no valid thread state was returned.
This then gave a rubbish stacktop of 0, which means unwinding
stops at first frame.

So, try harder to find a valid thread state when reporting the
host stacktrace.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16470