Sort out arm64 simulator issues

[stephen-hawley]

When building an iOS project in Xcode for simulator, you get arm64-apple-ios-simulator by default.
The problem we're seeing is that there is no *MIN_VERSION load command, but there is a BUILD_VERSION which we use instead of the MIN_VERSION. The problem is that the version that we get back has an OS and SDK version that is way different from the others, which causes us issues when we build against it.

We need to figure out how to make it build cleanly with arm64-apple-ios-simulator. It probably means tweaking the arguments to dynamically set the SDK information instead of using a static version.

[stephen-hawley]

I've cleaned up the code to handle the target quad and to compile code with "simulator", but a problem remains in that I don't know that we can build a fat version of XamGlue for iOS that can include arm64 and arm64-simulator. It raises questions like

• how does the dylib hold both arm64 and arm64-simulator since they're the same architecture?
• how is the Modules/XamGlue.swiftmodule directory laid out since it has files named arch.siwftmodule

[stephen-hawley]

It looks like you need to have an .xcframework and not a .framework

[spouliot]

FWIW the first mention of .xcframework I know of was done inside a Swift talk.
https://developer.apple.com/videos/play/wwdc2019/416/

The nice thing is that an xcframework is just another directory structure over the existing framework one.
https://github.com/spouliot/xcframework

[stephen-hawley]

When this is done, remember to remove the code the skips arm64