tools/include: adjust x86-specific population of xen/

tools/include: adjust x86-specific population of xen/

There's no need to use a shell loop construct here - ln's destination
can be specified as just the intended directory, as we don't mean to
change the names of any of the files. Also drop XEN_LIB_X86_INCLUDES for
having a single use only, and don't pass -f to ln, to allow noticing
name collisions right away.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Wei Liu <wl@xen.org>