[POWERPC] Fix zImage.coff on oldworld PowerMac

Recent changes to the PowerPC zImage wrapper broke zImage.coff due to
the addition of new ELF sections that aren't very well converted to
xcoff and not supported by old OpenFirmware. This fixes it by putting
those sections in the xcoff .data.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>