Roots for dynamic segments added with dload

[beta-ziliani]

Related #15 (for macOS)

dlopen adds various data/text segments that we need to add as roots. #24 fixes it for macOS, but a similar idea should be taken in place for other systems. See bdwgc/dyn_load.c for how to implement this in a variety of systems.

[ysbaddaden]

It means chasing references in library globals?

That's interesting, but that may be out of scope. For example we still wont know about library threads, so we can't scan their stack & CPU registers. If we expect all GC allocations to be referenced by the main program, then we don't need to scan the library segments & threads.