[mono] Implement support for UnmanagedCallersOnlyAttribute

[lambdageek]

The UnmanagedCallersOnlyAttribute is part of the C# FunctionPointers proposal, specifically it's applied to methods that will be called from native code only #32462 (but note that the calling convention part will be changing #37612)

Implementation note: the presence of the attribute means that ldftn should return a pointer to a native-to-managed wrapper.

[ghost]

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[lambdageek]

A related issue is to correctly parse the new "Unmanaged" (0x09) metadata calling convention. That's subject of #38480

[lambdageek]

Update actually this case will be a runtime exception. In CoreCLR since the UnmanagedCallersOnly check is done in the method itself (not at the ldftn site), it's hard for them to do the checking around a P/Invoke. Mono will follow CoreCLR behavior.

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Another implementation note: if the function is actually already a native function we should get a pointer to it without a wrapper.

[UnamangedCallersOnly]
[DllImport("SomeLib", EntryPoint="foo")]
public static extern void foo ();

In this case ldftn foo should still be a pointer to foo, not to a managed-to-native wrapper.

[lambdageek]

Implemented by #38728