openrisc: head: use THREAD_SIZE instead of magic constant

The stack size was hard coded to 0x2000, use the standard THREAD_SIZE
definition loaded from thread_info.h.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
[shorne@gmail.com: Added body to the commit message]
Signed-off-by: Stafford Horne <shorne@gmail.com>