Committed: Implement __gnat_set_close_on_exec on Windows

[Aaron W. LaFramboise]

 From <http://gcc.gnu.org/ml/gcc-patches/2005-03/msg02571.html>:

>> OK, how about these two patches?
> 
>> 2005-03-25  Aaron W. LaFramboise  <aaronavay62@aaronwl.com>
>> 
>> 	* adaint.c (_gnat_set_close_on_exec) [_WIN32]: Implement.
> 
> Assuming bootstrap and ada testing, OK.

Somehow this patch from two and a half years ago slipped through the 
cracks.  I've tested it and committed it.