Eliminate 'potential unsafe sign check of a bitwise operation' code defect

Eliminate 'potential unsafe sign check of a bitwise operation' code defect

* mark.c [!OS2] (GC_mark_from): Cast mark_stack_top-mark_stack to word
first, and cast bitwise OR operation result to signed_word, so that not
to apply bitwise OR operation to the value of ptrdiff_t.