xen: suppress abs symbol warnings for unused reloc pointers

arch/i386/xen/xen-asm.S defines some small pieces of code which are
used to implement a few paravirt_ops.  They're designed so they can be
used either in-place, or be inline patched into their callsites if
there's enough space.

Some of those operations need to make calls out (specifically, if you
re-enable events [interrupts], and there's a pending event at that
time).  These calls need the call instruction to be relocated if the
code is patched inline.  In this case xen_foo_reloc is a
section-relative symbol which points to xen_foo's required relocation.

Other operations have no need of a relocation, and so their
corresponding xen_bar_reloc is absolute 0.  These are the cases which
are triggering the warning.

This patch adds those symbols to the list of safe abs symbols.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Adrian Bunk <bunk@stusta.de>