gimli/libbacktrace don't handle dynamically loaded libraries

Currently iteration over all dynamic objects happens once in libbacktrace/gimli (gimli for sure, libbacktrace is a best approximation). This means that if a backtrace is generated and then afterwards a dynamic object is loaded (e.g. via dlopen) the backtraces through that new dynamic object may be missing symbol information. We'll likely need to add a fallback in gimli where if a symbol doesn't hit the cache of libraries we know about we check again to see if any new libraries have been opened.

Start by inspecting the gimli dynamic-object iteration and the library symbol cache, then compare how libbacktrace handles the same lookup. Reproduce the case where a library is loaded with dlopen after the initial scan; done means backtraces through the newly loaded library include symbol information.