Clean up arm64 tagged pointer handling.

- Tagged pointer handling is now out-of-process, multi-arch safe.
- The mask value is now validated at test time using the
  objc_debug_isa_class_mask variable documented by Greg Parker[1].
- Non-pointer ISAs are now detected using the LSB bit, also as
  documented by gparker.

[1] http://www.sealiesoftware.com/blog/archive/2013/09/24/objc_explain_Non-pointer_isa.html