gdb: update gnu ifunc resolve

gdb: update gnu ifunc resolve

Update elf_gnu_ifunc_resolve_by_cache() and elf_gnu_ifunc_resolve_by_got()
to use gdbarch_iterate_over_objfiles_in_search_order() in order to
restrict the objfile traversal to the initial namespace.

In order to extend this to other namespaces, we'd need to provide context,
e.g. via an objfile inside that namespace.