Blackfin arch: fix bug libstdc++ calling writev with an iovec containing { NULL, 0 } fails on Blackfin

Fix a problem reported in the forums - libstdc++ can call writev with an
iovec containing { NULL, 0 }, which works fine on i686-linux, but fails on
Blackfin.  Fixed by allowing size 0 transfers to/from userspace regardless
of the address.
Signed-off-by: Bernd Schmidt <bernd.schmidt@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>