also add BOOST_UNORDERED_DISABLE_NEON for users of libtester

[spoonincode]

Follow up to #519. I think we probably need to do this or run the risk of ODR violations (on ARM) if something else uses boost::unordered as part of the external test compilation. It's also possible it's already a ODR violation as-is since apply_context.hpp -- exposed as public libtester interface -- does a #include <boost/unordered/unordered_flat_map.hpp>. (I admit to not looking at unordered's impl yet to know if this ODR violation theory is possible or not, but seems like this is right thing to do -- it's possible original reporter in #517 will need it anyways to compile their external tests)

[ericpassmore]

Note:start
group: STABILITY
category: INTERNALS
summary: Add compile option BOOST_UNORDERED_DISABLE_NEON to better support arm.
Note:end