ada: Add support for symbolic backtraces with DLLs on Windows

ada: Add support for symbolic backtraces with DLLs on Windows

This puts Windows on par with Linux as far as backtraces are concerned.

gcc/ada/

	* libgnat/s-tsmona__linux.adb (Get): Move down descriptive comment.
	* libgnat/s-tsmona__mingw.adb: Add with clause and use clause for
	System.Storage_Elements.
	(Get): Pass GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT in the call
	to GetModuleHandleEx and remove the subsequent call to FreeLibrary.
	Upon success, set Load_Addr to the base address of the module.
	* libgnat/s-win32.ads (GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS): Use
	shorter literal.
	(GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT): New constant.