arc: rename aux.h to arc_aux.h

The goal is to clean-up Linux repository from AUX file names, because
the use of such file names is prohibited on other operating systems
such as Windows, so the Linux repository cannot be cloned and
edited on them.

Reviewed-by: Shahab Vahedi <list+bpf@vahedi.org>
Signed-off-by: Benjamin Szőke <egyszeregy@freemail.hu>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>