create GC TLS

[d-netto]

Encapsulates all relevant GC thread-local-state into a separate structure.

Motivation is that MMTk will have its own version of GC thread-local-state, so doesn't need all of the Julia GC TLS.

In the future, folks who would be using MMTk would be setting a pre-processor flag which would lead to either the stock Julia GC TLS or MMTk's GC TLS to be included in julia_threads.h.

I.e., we would have something like:

#ifdef MMTK_GC
#include <include/mmtk-gc-tls.h>
#else
#include "gc-tls.h"
#endif

where each of these files contains a GC TLS structure definition.

[kpamnany]

LGTM.