[PATCH/Ada] Make ZCX default on ppc-darwin

[Arnaud Charlet]

> OK? Bootstrapped and tested on powerpc-darwin with no
> regressions (and some Ada regressions fixed).

OK, thanks.

Arno

> ChangeLog:
> 
> 	* system-darwin-ppc.ads (ZCX_By_Default): Change to True.
> 	(GCC_ZCX_Support): Likewise.