Add local libc database provider for libcdb

[the-soloist]

Add local libc database provider for libcdb and a property context.local_libcdb on ContextType.

[peace-maker]

Thank you for starting to split #2259 up into manageable chunks! I really wanted to have it in 4.13.0 but didn't have the time to fix it up properly. I've left some comments inline.

[peace-maker]

Nice, thanks for documenting this!

[peace-maker]

Cool, I assume you'll use that libc and symbols in a future PR? We could just set the test dir to any valid folder, since it's not used I think. It might influence other tests, but only if we'd lookup the one libc that's in there.

[peace-maker]

All that's left is an entry in the changelog file :)

[the-soloist]

I will use them in future PRs, but I'm uncertain whether the test cases need modifications. I think I should remove them for now.