Fix aux_info corruption in rtld direct execution mode

[bdragon]

Summary

This manifests on powerpc64 as not being able to create threads in programs when using direct execution due to an invalid value being used for stack_prot, causing the kernel to reject mmap of the new thread stack.

The problem is that after

for (;; auxp++, auxpf++) {
    *auxp = *auxpf;
    if (auxp->a_type == AT_NULL)
            break;
}

is run, aux_info contains stale pointers to memory that has shifted slightly out from under it.
So, re-digest it to ensure the pointers are sane again.