create GC TLS (#55086)
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:
```C
#ifdef MMTK_GC
jl_gc_mmtk_tls_states mmtk_gc_tls;
#else
jl_gc_tls_states gc_tls;
#endif
```
