Remove PUSH_OBJ multi-line macro

Remove PUSH_OBJ multi-line macro
(code refactoring)

* finalize.c (GC_normal_finalize_mark_proc): Call GC_push_obj instead
of PUSH_OBJ.
* include/private/gc_pmark.h (PUSH_CONTENTS_HDR): Likewise.
* mark.c (GC_push_marked): Likewise.
* mark.c [ENABLE_DISCLAIM] (GC_push_unconditionally): Likewise.
* finalize.c (GC_normal_finalize_mark_proc): Remove hhdr local
variable.
* include/private/gc_pmark.h (PUSH_OBJ): Transform macro to inline
function; rename to GC_push_obj; update comment; return mark_stack_top.