[boehm] Put *_freelists into thread_local_freelists (as in BDWGC v7)

[boehm] Put *_freelists into thread_local_freelists (as in BDWGC v7)

Mono create_allocator accesses libgc private *_freelists directly.
These fields have been grouped together into struct
thread_local_freelists in BDWGC v7.
This commit does code refactoring to match the structure of external
GC regarding *_freelists.

 * Move GRANULARITY, NFREELISTS (used by *_freelists) out of
   GC_Thread_Rep.

 * Move ptrfree_freelists, normal_freelists, gcj_freelists from
   GC_Thread_Rep to struct thread_local_freelists.

 * Add tlfs field (of type struct thread_local_freelists) to
   GC_Thread_Rep.

 * Update pthread_support.c and boehm-gc.c code accordingly.