tests/libexec/ld.elf_so: Fix helper library makefiles.

1. Consolidate logic into a single helper.mk to reduce duplication.
2. Set NO* variables, not MK* variables which are reserved for user.
3. Avoid eager X!= in favour of lazy ${X:sh}.
4. Mark _g.a set list entries obsolete. Never should've been built!

PR misc/57462