Change backtrace-specific code to comply with C++11 standard

Change backtrace-specific code to comply with C++11 standard

Issue #206 (bdwgc).

* dbg_mlc.c (GC_get_back_ptr_info): Cast GC_REVEAL_POINTER() result
to ptr_t (when assigned to bp local variable).
* dbg_mlc.c (GC_generate_random_valid_address): Cast result of
GC_generate_random_heap_address() to ptr_t.
* dbg_mlc.c (GC_print_backtrace): Cast GC_print_heap_obj() argument
to ptr_t.