FreeBSD: implement support for pthread synchronization primitives

[RalfJung]

We currently don't support the pthread synchronization primitives on FreeBSD. And it turns out that supporting them will require some refactoring of how we implement those primitives: on FreeBSD, pthread_mutex_t et al are just a pointer to the actual data. That means on a 32bit system they are only 4 bytes large, which is not large enough for us to store the data we need to store directly in the pthread_mutex_t.

So we need to do what likely the FreeBSD implementation also does, and allocate some memory to store the mutex data in, making the mutex itself just a pointer to that memory. We could do that only on FreeBSD and keep things unchanged on all other targets, but I see no reason to carry two implementations -- let's just switch everything over. That will also give us detection of leaks or double-frees for these constructs, since we'd leak / double-free their backing memory.

The static initializer is a null pointer, so all mutex operations will have to lazily allocate the backing memory. (That can replace the current scheme of lazily assigning a mutex ID.) This has to happen atomically since multiple threads could be trying to acquire the same lock that was not previously initialized.

There's a little complication on Linux, which has static initializers for non-default mutex kinds. The lazy allocation will have to carry over the mutex kind to the newly allocated memory.