[NativeAOT] Support exporting methods from referenced assemblies in a formal way

[ivanpovazan]

This PR adds support for exporting methods from referenced assemblies in a more controllable way.

The changes include:

1. UnmanagedEntryPointsAssembly - new ItemGroup which can be used to include names of assemblies for which unmanaged entry points have to be exported
2. --generateunmanagedentrypoints- new ILCompiler switch which can be used to pass the list of assemblies for which methods have to be exported and is populated with items from 1.

3. Test case which verifies the added functionality
4. Exporting unmanaged entry points from the system module is now achieved by explicitly including System.Private.CoreLib in 1.
5. Includes --export-dynamic native linker switch in the build, when it is required to dynamically access exported symbols from the main executable targeting linux systems (more information here)

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Fixes #65755

[ghost]

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Issue Details

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This PR adds support for exporting methods from referenced assemblies in a more controllable way.

The changes include:

• new ItemGroup: IlcRootedAssembliesExports which can be used to include names of assemblies for which methods have to be exported
• new ILCompiler switch: --rootedassembliesexports which can be used to pass the list of assemblies for which methods have to be exported (and is populated with the above-mentioned ItemGroup)
• the feature is enabled in the library and CustomNativeMain compilation modes.
• test case which verifies the added functionality

NOTE: It is possible that the naming is not ideal and I am open for any suggestions.

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Fixes #65755

Author:	ivanpovazan
Assignees:	ivanpovazan
Labels:	area-NativeAOT-coreclr
Milestone:	-

[jkotas]

LGTM. Thank you!