[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  <ebotcazou@adacore.com>

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

-- 
Eric Botcazou
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