Export GC_is_tmp_root() and GC_print_trace[_inner]()

Export GC_is_tmp_root() and GC_print_trace[_inner]()

Note: these 3 functions are not used by GC itself.

* include/gc_mark.h (GC_is_tmp_root, GC_print_trace,
GC_print_trace_inner): New public API prototype.
* mark.c [TRACE_BUF] (GC_print_trace_inner, GC_print_trace): Define as
public (GC_API+GC_CALL).
* mark_rts.c [MSWIN32 && !NO_DEBUGGING] (GC_is_tmp_root): Likewise.
* mark_rts.c [MSWIN32 && !NO_DEBUGGING] (GC_is_tmp_root): Replace
GC_bool return type with int, replace ptr_t argument type to void*
(because GC_bool and ptr_t are not exported from GC).