powerpc/boot: Add support for 64bit little endian wrapper

The code is only slightly modified : entry points now use the
FIXUP_ENDIAN trampoline to switch endian order. The 32bit wrapper
is kept for big endian kernels and 64bit is enforced for little
endian kernels with a PPC64_BOOT_WRAPPER config option.

The linker script is generated using the kernel preprocessor flags
to make use of the CONFIG_* definitions and the wrapper script is
modified to take into account the new elf64ppc format.

Finally, the zImage file is compiled as a position independent
executable (-pie) which makes it loadable at any address by the
firmware.

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>