[clr-ios] Implement Crossgen2 Mach-O object writer

[kotlarmilos]

Description

Implement Mach-O output in Crossgen2 as a parallel implementation to the existing PE writer. The Mach-O writer should generate composite ReadyToRun (R2R) images for Apple platforms, mapping R2R sections and symbols to Mach-O segments and sections. It should also expose the R2R header via an exported symbol and emit native debug information together with native code.

The work includes enforcing composite-only mode and rejecting single-file ReadyToRun images to avoid the complexity of exporting per-assembly symbols. This requires adding R2R-specific features to the MachObjectWriter and mapping R2R sections and symbols to Mach-O segments and sections, including native code in __TEXT,__text, fixups and indirections in a __DATA section, and exposing the R2R header via an exported symbol. Crossgen2 targets must be updated to link the Mach-O object into a dylib, using the existing macOS triple for Apple mobile. Native debug information should be emitted alongside the native code, and R2RDump should be extended to support dumping Mach-O R2R images.

The progress is tracked in #120065

DoD: Running Crossgen2 with --targetos macos--targetarch arm64 --composite --obj-format macho generates a valid Mach-O object file containing code and an embedded R2R header.