libc: Make elf_aux_info() return an error if AT_USRSTACK* is undefined

Otherwise we do not fall back to sysctls if the auxv entries are not
defined by the kernel. Arguably this is not a bug since we do not
support newer libc running on an older kernel, but we can be a bit more
gentle for the benefit of Valgrind or any other software which
synthesizes the auxv for virtualization purposes.

Reported by: Paul Floyd <paulf2718@gmail.com>
MFC after: 1 week
Reviewed by: brooks, kib
Differential Revision: https://reviews.freebsd.org/D37036