rtld: Set foundation to use aux vectors to get osreldate

rtld: Set foundation to use aux vectors to get osreldate

__getosreldate isn't used by rtld currently.  The FreeBSD dynamic linker
does use it indirectly in libc's mmap wrapper.  The capability is included
here both to ease the task to synchronizing with FreeBSD, and to quickly
obtain the capability in the future if necessary.  It's currently disabled
with the undefined ENABLE_OSRELDATE macro.

The splitting of digest_dynamic required to use aux vectors also has the
benefit of avoiding the use of malloc at startup.