powerpc/64: Add an option to force run-at-load to test relocation

This adds a config option that can help exercise the case when
the kernel is not running at PAGE_OFFSET.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>