Mac/iOS support, including aarch64 port

Supersedes #19.

As before: rewrites the Xcode project, as referenced in issue #28. Includes fixes for ARM-32 targets under newer LLVM assemblers and uses unified syntax.

Integrates AArch64 support for this year's crop of iOS devices. I'd like to see some regression testing from the ARM/Linaro folk against legitimate AArch64 devices. It's working for me, but the changes were mostly workarounds for compiler errors aped from the ARM port.