Powerpc: separate CONFIG_RELOCATABLE from CONFIG_CRASHDUMP in boot code

Fix head_64.S so that we can build a relocatable kernel
that isn't necessarily a crash-dump kernel

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Sonny Rao <sonnyrao@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>