Fix warnings in gc-mmtk.c

[qinsoon]

Fix the following warnings:

/home/yilin/Code/julia_workspace/julia/src/gc-mmtk.c: In function 'jl_gc_scan_julia_exc_obj':
/home/yilin/Code/julia_workspace/julia/src/gc-mmtk.c:903:49: warning: initialization of 'jl_value_t **' {aka 'struct _jl_value_t **'} from incompatible pointer type 'union <anonymous> *' [-Wincompatible-pointer-types]
  903 |                     jl_value_t** new_obj_slot = &bt_entry[2 + jlval_index].jlvalue;
      |                                                 ^
/home/yilin/Code/julia_workspace/julia/src/gc-mmtk.c:911:43: warning: initialization of 'jl_value_t **' {aka 'struct _jl_value_t **'} from incompatible pointer type 'union <anonymous> *' [-Wincompatible-pointer-types]
  911 |             jl_value_t** stack_obj_slot = &stack_raw[itr-1].jlvalue;
      | 

[udesou]

LGTM.