ARC: export clear_user_page() for modules

0day bot reports a build error:
  ERROR: modpost: "clear_user_page" [drivers/media/v4l2-core/videobuf-dma-sg.ko] undefined!
so export it in arch/arc/ to fix the build error.

In most ARCHes, clear_user_page() is a macro. OTOH, in a few
ARCHes it is a function and needs to be exported.
PowerPC exported it in 2004. It looks like nds32 and nios2
still need to have it exported.

Fixes: 4102b533 ("ARC: [mm] Aliasing VIPT dcache support 2/4")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: linux-snps-arc@lists.infradead.org
Signed-off-by: Vineet Gupta <vgupta@kernel.org>

arch/arc/mm/cache.c

@@ -1123,7 +1123,7 @@ void clear_user_page(void *to, unsigned long u_vaddr, struct page *page)
 	clear_page(to);
 	clear_bit(PG_dc_clean, &page->flags);
 }
-
+EXPORT_SYMBOL(clear_user_page);
 
 /**********************************************************************
  * Explicit Cache flush request from user space via syscall