Remove redundant GC_with_callee_saves_pushed call in multi-threaded builds

Remove redundant GC_with_callee_saves_pushed call in multi-threaded builds

In GC_push_regs_and_stack, if cold_gc_frame is null and threads
support is on then there is no need to call
GC_with_callee_saves_pushed(GC_push_current_stack) as
GC_push_current_stack is no-op in this case.

* mark_rts.c (GC_push_current_stack): Reformat title comment.
* mark_rts.c [THREADS] (GC_push_current_stack): Do not check that
cold_gc_frame is non-zero; add comment about it.
* mark_rts.c [THREADS] (GC_push_regs_and_stack): If cold_gc_frame is
null then do not call GC_with_callee_saves_pushed().