[POWERPC][XEN] Fix yaboot loading and other linker script tweaks.

[POWERPC][XEN] Fix yaboot loading and other linker script tweaks.
yaboot refuses to load segments at file offset 0, so move the ELF headers out
of our text segment. Also hardcode the link address in the linker script, and
remove the now-inaccurate start/end comments.
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
--HG--
extra : transplant_source : %A4%1B%8C%D0%1A%D2%87%D6%0A%5EY%CC%FF%A2%3B%5D%D8%83z%F8