erlang: use libatomic_ops if available

Erlang can either provide its own atomic_ops implementation or use the
one in libatomic_ops. This patch ensures that it only uses the one from
libatomic_ops if it is available.

Fixes:

http://autobuild.buildroot.net/results/3eb/3ebfd36948d5a642cc83ce31ddf5902cb369bfbd/
http://autobuild.buildroot.net/results/04c/04cbf79ccf159e15f9ea3953b425faadafc8a1c0/
http://autobuild.buildroot.net/results/fc9/fc9904446553e67dd44954310fea26d751a0df5d/
http://autobuild.buildroot.net/results/501/501a12da25615071fb6197a898456cfe41a6451d/
http://autobuild.buildroot.net/results/34e/34e2fbb4d39737fd7defa5bd607726d52ef38568/
http://autobuild.buildroot.net/results/722/7220c3905ebe9e081f1e3b90641b0859d996c2ac/
http://autobuild.buildroot.net/results/ffd/ffdc531d8cf9adb43340d045aa88eb0b39fc9bb7/
http://autobuild.buildroot.net/results/b4f/b4f238bcb30b0191ce1a2be669fa792c8ccd3e69/



Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>