[PATCH, Ada, Darwin, PPC] PPC Darwin has stack check probes.

On PPC, Darwin uses the same code as other parts of the port.

2019-07-02  Iain Sandoe  <iain@sandoe.co.uk>

	* libgnat/system-darwin-ppc.ads: Set Stack_Check_Probes True for
	PPC Darwin.

From-SVN: r272947