[PATCH][ARM64] Fix predefines and add plumbing for -mfpu=

[Tim Northover]

Hi James,

Mostly looks OK, though it might be better off as a couple of commits:
I can see combining predefines & BE, but NEON looks a bit peripheral.

+    case llvm::Triple::NetBSD:
+      return new NetBSDTargetInfo<AArch64leTargetInfo>(Triple);

Shouldn't that be ARM64 for now? (& Big-endian case).

Also, isn't there some AArch64 predefine test you can hook into to
make sure we actually are emitting the right ones? It looks like all
of your test changes are just of the "-target-feature neon" kind.

Cheers.

Tim.