[Ada] Do not enable atomic primitives on Power/Darwin 32-bit

[Eric Botcazou]

Iain Sandoe privately reported that the overhaul of the system files had 
broken the Ada compiler on Power/Darwin 32-bit.  Applied on the mainline.


2016-12-05  Eric Botcazou  <ebotca...@adacore.com>

        * system-darwin-ppc.ads (Support_Atomic_Primitives): Set to True only
        if the word size is 64.

-- 
Eric Botcazou
Index: system-darwin-ppc.ads
===================================================================
--- system-darwin-ppc.ads	(revision 243172)
+++ system-darwin-ppc.ads	(working copy)
@@ -161,7 +161,7 @@ private
    Stack_Check_Probes        : constant Boolean := False;
    Stack_Check_Limits        : constant Boolean := False;
    Support_Aggregates        : constant Boolean := True;
-   Support_Atomic_Primitives : constant Boolean := True;
+   Support_Atomic_Primitives : constant Boolean := Word_Size = 64;
    Support_Composite_Assign  : constant Boolean := True;
    Support_Composite_Compare : constant Boolean := True;
    Support_Long_Shifts       : constant Boolean := True;