enabling caching for dl_iterate_phdr()

[Roland McGrath]

> David Mosberger wrote:
> 
> > In the meantime, it occurred to me that we can get rid of one of the
> > "unsigned long long" variables by taking advantage of the fact that
> > "_dl_nloaded == _dl_load_adds - _dl_load_subs"
> 
> _dl_nloaded is not available outside ld.so.

It's part of rtld_global, just like _dl_load_adds is.
We are only talking about dl_iterate_phdr's implementation here.