ensure microblaze __set_thread_area returns success

since it did not set the return-value register, the caller could
wrongly interpret this as failure.

src/thread/microblaze/__set_thread_area.s

 .global __set_thread_area
 .type   __set_thread_area,@function
 __set_thread_area:
-	rtsd     r15, 8
 	ori      r21, r5, 0
+	rtsd     r15, 8
+	ori      r3, r0, 0