Workaround more 'void pointers in calculations' cppcheck warnings

Workaround more 'void pointers in calculations' cppcheck warnings

* include/new_gc_alloc.h (single_client_gc_alloc_template::allocate,
single_client_gc_alloc_template::ptr_free_allocate,
single_client_gc_alloc_template::deallocate,
single_client_gc_alloc_template::ptr_free_deallocate,
single_client_traceable_alloc_template::allocate,
single_client_traceable_alloc_template::ptr_free_allocate,
single_client_traceable_alloc_template::deallocate,
single_client_traceable_alloc_template::ptr_free_deallocate): Replace
GC_Xobjfreelist_ptr+nwords with &GC_Xobjfreelist_ptr[nwords].
* reclaim.c (GC_start_reclaim): Replace fop++ with (*(word**)&fop)++.
* thread_local_alloc.c (return_freelists): Replace gfl+i with &gfl[i].