[Ada] Disable the creation of the main task secondary stack by the binder

Users can now specify that the binder should not create a secondary stack for
the main (environment) task through the binder switch -Q0. This is useful for
ZFP runtime users who allocate secondary stacks for their application
themselves.

2018-05-25  Patrick Bernardi  <bernardi@adacore.com>

gcc/ada/

	* switch-b.adb (Scan_Binder_Switches): binder switch -Q accepts Natural
	numbers.

From-SVN: r260731

gcc/ada/ChangeLog

+2018-05-25  Patrick Bernardi  <bernardi@adacore.com>
+
+	* switch-b.adb (Scan_Binder_Switches): binder switch -Q accepts Natural
+	numbers.
+
 2018-05-25  Doug Rupp  <rupp@adacore.com>
 
 	* tracebak.c (aarch64-linux): Implement.

gcc/ada/switch-b.adb

@@ -399,7 +399,7 @@ package body Switch.B is
             end if;
 
             Ptr := Ptr + 1;
-            Scan_Pos
+            Scan_Nat
               (Switch_Chars, Max, Ptr,
                Quantity_Of_Default_Size_Sec_Stacks, C);