To accomodate cases where objects allocated on the secondary stack
needed a more constrained alignement than Standard'Maximum_Alignement,
the alignment for all allocations in the full runtime were forced on to
be aligned on Standard'Maximum_Alignement*2. This changes removes this
workaround and correctly handles the over-alignment in all runtimes.

This change modifies the SS_Allocate procedure to accept a new Alignment
parameter and to dynamically realign the pointer returned by the memory
allocation (Allocate_* functions or dedicated stack allocations for
zfp/cert).

It also simplifies the 0-sized allocations by not allocating any memory
if pointer is already correctly aligned (already the case in cert and
zfp runtimes).

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

gcc/ada/

	* libgnat/s-secsta.ads (SS_Allocate): Add new Alignment
	parameter.
	(Memory_Alignment): Remove.
	* libgnat/s-secsta.adb (Align_Addr): New.
	(SS_Allocate): Add new Alignment parameter. Realign pointer if
	needed. Don't allocate anything for 0-sized allocations.
	* gcc-interface/utils2.cc (build_call_alloc_dealloc_proc): Add
	allocated object's alignment as last parameter to allocation
	invocation.