Fix mark stack excessive growth during parallel mark
(The growth observed in applications that implement things like weak
hash tables and iterate over the members marking entries using the
supplied GC_MARK_AND_PUSH macro.)

When overflow is signaled, only set GC_mark_stack_too_small if we
are using the global mark stack.  In parallel mode, local mark stack
is used, so the global mark stack is grown by GC_return_mark_stack
mainly.

* mark.c (GC_signal_mark_stack_overflow): Do not set
GC_mark_stack_too_small if GC_parallel.