ppc64/kdump: Fix ELF header endianess

The ELF header created among the loading of the kdump kernel should be
flagged using the current endianess and not always as big endian.

Without this patch the data exposed in /proc/vmcore are not readable when
running in LE mode.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Simon Horman <horms@verge.net.au>