external toolchain: also copy the libthread_db.so for gdbserver

gdbserver dlopen(3)s libthread_db.so at runtime, so there is no
dependency on it (does not appear as being (NEEDED)).

Copy libthread_db.so from external toolchain when gdbserver is enbled.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>