package/nginx: add libatomic_ops support

Force the use of libatomics_ops on the sparc v8 and leon3 platforms to
allow nginx to build.  Optionally use libatomic_ops on other platforms.

This fixes:
 http://autobuild.buildroot.net/results/20dc41cd62887222d2569d81b5865893baeafaae/
 http://autobuild.buildroot.net/results/eb47af0c8df6fae82859865fb8b9912f45960444/



Cc: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>