Make symbol visibility explicit in our test/demo BMI modules

[PhilMiller]

See comments here

https://gcc.gnu.org/wiki/Visibility

https://gist.github.com/ax3l/ba17f4bb1edb5885a6bd01f58de4d542

Current behavior

We're setting a compiler flag to make all symbols in the relevant modules visible

Expected behavior

Modules have the symbols actually necessary for BMI operation marked as exported, and everything else left to defaults.

[mattw-nws]

Question... is the explicit visibility marking something that makes more sense in bmi.hxx? Would that work? Or would we have to mark the same methods explicitly in the concrete subclass anyway?

[PhilMiller]

I'm pretty sure they'll be necessary in bmi.hxx and bmi.h regardless. I don't know whether that would then spare us a need to mark anything in the implementations as well. I think it would, since the only symbols that need to be exported are the ones that might get looked up by name, not those that go through virtual dispatch or function pointers, since the compiler already gave us the addresses in those.