selftests/powerpc/cache_shape: Fix out-of-tree build

Use TEST_GEN_PROGS and don't redefine all, this makes the out-of-tree
build work. We need to move the extra dependencies below the include
of lib.mk, because it adds the $(OUTPUT) prefix if it's defined.

We can also drop the clean rule, lib.mk does it for us.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>