rtld: Add MD_OBJ_ENTRY to extend Struct_Obj_Entry

Add a macro the architectures can use to add per-arch fields to
Struct_Obj_Entry.

Reviewed by: kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45116

(cherry picked from commit 06db20ffeca9898e5802d63f3b06caaa37c3a4ed)