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

[markj]

Summary

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 here.

I assume here that the kernel-provided values for AT_USRSTACK* are
always non-zero.

Reported by: Paul Floyd

[kib]

Ok, timekeep uses the similar check. I wonder should we record the auxv presence in init_aux(), instead.