ARM MachO: sort out isTargetDarwin/isTargetIOS/... checks.

The ARM backend has been using most of the MachO related subtarget
checks almost interchangeably, and since the only target it's had to
run on has been IOS (which is all three of MachO, Darwin and IOS) it's
worked out OK so far.

But we'd like to support embedded targets under the "*-*-none-macho"
triple, which means everything starts falling apart and inconsistent
behaviours emerge.

This patch should pick a reasonably sensible set of behaviours for the
new triple (and any others that come along, with luck). Some choices
were debatable (notably FP == r7 or r11), but we can revisit those
later when deficiencies become apparent.