ARM: convert isTargetIOS checks to isTargetDarwin.

The distinction is mostly useful in the front-end. By the time we get here,
there are very few situations where we actually want different behaviour for
Darwin and IOS (in fact Darwin mostly just exists in a few tests). So this
should reduce any surprising weirdness for anyone using it.

No functional change on anything anyone actually cares about.

llvm-svn: 224035