microblaze: Setup correct pointer to TLS area

Setup a pointer to the TLS area in copy_thread.
r10 is 6th argumetn which contains TLS area.
And r21 is the thread reg.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: David Holsgrove <david.holsgrove@petalogix.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>