### Description
This PR enables building app extensions with NativeAOT.
App extensions are class libraries and to build them with NativeAOT we
must not specify `CustomNativeMain=true`. If we do, ILC would expect
that the input assembly has a managed Main as the module entry point.
Additionally, when building class libraries (with the absence of a
managed Main entry point), our static reference from:
2e5ef1eb1c/runtime/nativeaot-bridge.m (L39)
requires build-time symbol resolution. To avoid linking errors, we
generate an empty `__managed__Main`
in the native bootstrapping code of the app extension (e.g., in
`main.arm64.mm`).
### Testing
The unit tests have been introduced to test building app extensions with
both Mono and NativeAOT.
Executing an iOS app TodayExtension built with NativeAOT has been
verified manually on an actual device.
---
Fixes: https://github.com/xamarin/xamarin-macios/issues/20653
This PR resolves a crash when running the linker on publishing iOS extensions.
The crash would occur here in failing to resolve corelib.
The reason this would fail was System.Private.CoreLib.dll was not in input_assemblies.
This was because we were passes the set of reference assemblies not the expected 'real' ones, and those do not include CoreLib.
After a bunch of digging, this was because _ComputeManagedRuntimePackAssembliesIfSelfContained target was not being set as a condition of _ComputeAssembliesToPostprocessOnPublish.
_ComputeManagedRuntimePackAssembliesIfSelfContained happened to be the place these were added, and wasn't being set since it has a condition of $(SelfContained) == 'true'
Now confusingly SelfContained WAS being set to true, but only in the targets file, which was too late, as it was checked in a 'global' property group outside of a target.
This means we'd fail to set SelfContained until after the condition, and not run.
This was verified by setting /p:SelfContained=true to true.
I also looked at removing the condition above, since https://github.com/dotnet/runtime/issues/54406 is fixed, however this caused project that didn't set RuntimeIdentifier to fail.
Ivan Povazan
Rolf Bjarne Kvinge
Chris Hamons