target-finalize: do not strip libthread_db.so

For proper threading debug support, the libthread_db.so library cannot
be stripped on the target.  This is because the target gdbserver will
also load up this library at runtime and poke around its symbols.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>