ada: Ensure Default_Stack_Size is greater than Minimum_Stack_Size

ada: Ensure Default_Stack_Size is greater than Minimum_Stack_Size

The Default_Stack_Size function does not check that the binder specified
default stack size is greater than the minimum stack size for the runtime.
This can result in tasks using default stack sizes less than the minimum
stack size because the Adjust_Storage_Size only adjusts storages sizes for
tasks that explicitly specify a storage size. To avoid this, the binder
specified default stack size is round up to the minimum stack size if
required.

gcc/ada/

	* libgnat/s-parame.adb: Check that Default_Stack_Size >=
	Minimum_Stack_size.
	* libgnat/s-parame__rtems.adb: Ditto.
	* libgnat/s-parame__vxworks.adb: Check that Default_Stack_Size >=
	Minimum_Stack_size and use the proper Minimum_Stack_Size if
	Stack_Check_Limits is enabled.