Support for thread-local variables / `pthread_getspecific` & `pthread_setspecific`

[michael-schwarz]

The notion of C11 thread-local variables seems to map onto pthread_getspecific & pthread_setspecific. So when adding support for C11, support for this should be added to.

[sim642]

Besides C11's thread-local specifier there is also GCC's __thread that we found with @vesalvojdani in silver searcher. It should be very easy to just ignore races on thread-local things.

[michael-schwarz]

I think the good first issue applies mostly to not reporting races for these variables, the support to privatize the values is a bit trickier - one has to think a bit what happens when thread ids are non-unique (it would seem like that should also work, but some thought is needed here).

[sim642]

#1071 handles the race checking of __thread, but there's still C11's _Thread_local for which the same could apply. It probably requires changes in CIL as well: goblint/cil#13.