x86/link: Discard the alternatives ".discard" sections

This appears to have been missed when porting the alternatives framework from
Linux, and saves us a section which is otherwise loaded into memory.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>

xen/arch/x86/xen.lds.S

@@ -197,6 +197,8 @@ SECTIONS
        *(.exit.text)
        *(.exit.data)
        *(.exitcall.exit)
+       *(.discard)
+       *(.discard.*)
        *(.eh_frame)
 #ifdef EFI
        *(.comment)