linux-user: save auxv length

linux-user: save auxv length

We create our own AUXV segment on stack and save a pointer to it.
However we don't save the length of it, so any code that wants to
do anything useful with it later on has to walk it again.

Instead, let's remember the length of our AUXV segment. This
simplifies later uses by a lot.

(edited by Riku to apply to qemu HEAD)

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>