.exit.text is discarded at run time and there are some references from
that to .exit.data, so we need to discard .exit.data at run time as well.

Fixes these errors:

`.exit.data' referenced in section `.exit.text' of drivers/built-in.o: defined in discarded section `.exit.data' of drivers/built-in.o
`.exit.data' referenced in section `.exit.text' of drivers/built-in.o: defined in discarded section `.exit.data' of drivers/built-in.o
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>