uprobes/x86: Move default_xol_ops's data into arch_uprobe->def

Finally we can move arch_uprobe->fixups/rip_rela_target_address
into the new "def" struct and place this struct in the union, they
are only used by default_xol_ops paths.

The patch also renames rip_rela_target_address to riprel_target just
to make this name shorter.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jim Keniston <jkenisto@us.ibm.com>