Arm64 for iOS Simulator

Now that there are plenty of Apple Silicon macs in the wild, the recommended architectures for building for iOS are:

- **arm64** (devices)
- **x86_64** and **arm64** (simulator)

I tried to modify the iphonesim toolchain XML to make it build the arm64 architecture when `HXCPP_ARM64` is defined, but at the end of the build process, Xcode is complaining that my arm64 binary is made for iphoneos and not iphonesimulator.

Wondering if I'm missing something, so opening this issue:

- Is there anything else other than the iphonesim XML toolchain that should be modified to make this work? Like, are there checks in the HXCPP codebase somewhere that would assume ARM64 on iOS equals device (which was true in the past) that could break the simulator build?

I am willing to try to make those changes and make a pull request for this, but if you have any pointers to what should be reviewed to make these changes I'm all ears (@hughsando).