nginx: disallow AIO support on AArch64

The nginx AIO support cannot build on AArch64, because it hardcodes
the usage of SYS_eventfd(), while it should just use eventfd() from
the C library.

See https://bugs.launchpad.net/linaro-aarch64/+bug/1160013 for details.

Fixes:

  http://autobuild.buildroot.org/results/54b/54bb761f9e774d91066b78217efcd583fdd85ff3/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>