powerpc: discard .exit.data at runtime

.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>