package/nginx: disallow AIO support on arc

The nginx AIO support cannot build on arc, because it hardcodes
the usage of SYS_eventfd(), while it should just use eventfd() from
the C library.

Fixes:
  http://autobuild.buildroot.org/results/f3e/f3e2c7a937517a5444f8c6cc0b02692af897b3e0/



[Thomas: adjust comment as suggested by Yann.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>