[Ada] Remove 2GB secondary stack limit for 64-bit processors

[Ada] Remove 2GB secondary stack limit for 64-bit processors

This patch removes the restriction introduced recently that limited the size
of the secondary stack to 2GB. The size of the secondary stack is now limited
to half of the size of the memory address space for the target.

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

gcc/ada/

	* libgnat/s-parame.ads, libgnat/s-parame__vxworks.ads,
	libgnat/s-parame__ae653.ads, libgnat/s-parame__hpux.ads (Size_Type):
	Expand range of type to match the address space of the target.
	(Task_Storage_Size): Remove unused type.

gcc/testsuite/

	* gnat.dg/sec_stack1.adb: New testcase.

From-SVN: r260742