Attached is an (ugly but relatively simple) patch that allows
Xenlinux/ia64 to be built using the linux "-sparse" tree
methodology (once I populate some files in the -sparse tree).

The changes are not dramatic and I think they can be done
much more cleanly, but I'm not a build expert so would
appreciate improvements from those more comfortable with
the Xen build environment... and perhaps it can be
made more generic so the ppc guys can benefit too.

The primary changes are:
1) wherever a make is done with ARCH=xen, this needs
   to be converted to ARCH=ia64.  (This can probably
   be done as ARCH=$(XENARCH) or something like that
   but I'm not sure how to do it without requiring x86
   and x86_64 builds to specify XENARCH= at build time.)
2) A sequence of mkbuildtree "massage" commands need to
   be executed before mkbuildtree does its linking thing.
   Perhaps this could be done as:
    if -e mkbuildtree.prelink.$XENARCH
        mkbuildtree.prelink.$XENARCH
   Though ia64 doesn't need it, maybe also add a "postlink"
   equivalent?

There is also one change because the ia64 defconfig filename
is different.  I can change this filename to conform with
the existing convention but perhaps the defconfig pathname
could be set as part of the prelink file?

Could someone with the necessary build knowledge make
the improvements and commit this please? Let me know if/when
it goes in and I will populate the ia64 -sparse branches.

Thanks,
Dan