fix sh __set_thread_area uninitialized return value

this caused the dynamic linker/startup code to abort when r0 happened
to contain a negative value.

src/thread/sh/__set_thread_area.s

 .global __set_thread_area
 .type   __set_thread_area, @function
 __set_thread_area:
+	ldc r4, gbr
 	rts
-	 ldc r4, gbr
+	 mov #0, r0