Support Mach-O as a container for R2R images

[agocke]

This has two separate paths: MacOS and iOS. This is because MacOS allows for arbitrary page protection changes (JIT entitlement), while iOS doesn't. That means that MacOS and iOS can use different implementations. Eventually, we should converge to one, the iOS representation, but we can get started by simply using Mach-O as a container of PE images.

• Refactor and include Mach-O object writer from NAOT in R2R (Use an ObjectWriter-based PE writer for crossgen2 #120454)
• Make minimal modifications necessary to load R2R images from Mach-O images
  • Design Mach-O format
  • Implement writing in crossgen2
  • Implement loading in CoreCLR
• Make modifications necessary for R2R images on iOS
  • Design full Mach-O format
  • Implement in crossgen2
  • Implement loading in CoreCLR
  • Implement loading in the host for iOS
• Composite image support

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[janvorli]

While designing this, it would be good to keep in mind that long term, it would be great to also use a native R2R format for Linux.