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

[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