xen/ppc: Ensure ELF sections' physical load addresses start at 0x0

xen/ppc: Ensure ELF sections' physical load addresses start at 0x0

Some boot mechanisms, including the new linux file_load kexec systemcall
used by system firmware v2.10 on RaptorCS systems will try to honor the
physical address field of the ELF LOAD section header, which will fail
when the address is based off of XEN_VIRT_START (0xc000000000000000).

To ensure that the physical address of the LOAD section header starts at
0x0, move x86's DECL_SECTION macro to xen.lds.h and use it to declare
all sections.

Signed-off-by: Shawn Anastasio <sanastasio@raptorengineering.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>