tools/nolibc: add auxiliary vector retrieval for arm

In the _start block we now iterate over envp to find the auxiliary
vector after the NULL. The pointer is saved into an _auxv variable
that is marked as weak so that it's accessible from multiple units.

Signed-off-by: Willy Tarreau <w@1wt.eu>

It was tested in arm, thumb1 and thumb2 modes.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>