[POWERPC] Export copy_page() on 32bit

Export copy_page() on 32-bit powerpc; unionfs needs it.

Unionfs already builds as a module on 64bit powerpc, so the export is
placed within an existing CONFIG_PPC32 #ifdef.

Signed-off-by: Joseph Fannin <jfannin@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>