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

[Pierre-Marie de Rodat]

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.

Tested on x86_64-pc-linux-gnu, committed on trunk

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

gcc/ada/

        * switch-b.adb (Scan_Binder_Switches): binder switch -Q accepts Natural
        numbers.
--- gcc/ada/switch-b.adb
+++ 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);