Merge pull request #5299 from cherusker/cherusker-2017-08-01-next-generic-inst-id

[metadata] Fix racy next_generic_inst_id

Like suggested by @luhenry, I would like to propose this fix which was discussed in https://bugzilla.xamarin.com/show_bug.cgi?id=58423. As `next_generic_inst_id` is `int`, `InterlockedIncrement64 ()` seems to be the right choice.

Also, I cannot add any reviewers or assignees, which is why I would kindly ask @lambdageek to review this as well :)