docs: correct supported CPU targets

[micahcooley]

Corrects the README target claim to reflect the supported experimental ARM64 target and the absence of RISC-V support.

The prior table said all multi-architecture CPU support was unavailable even though the ARM64 backend is compiled, tested, and self-hosted through the v1 release gates.

Validation: ARM64 probe emitted static AArch64 ELF and exited 42 under qemu-aarch64-static; tests/run_native_arm64.sh passed 99/99.

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[gemini-code-assist]

Code Review

This pull request updates the README.md to document the addition of experimental ARM64 Linux ELF cross-compilation support in the v1 compiler. It includes updated build target examples, details on verifying ARM64 output via qemu-user, and updates the feature matrix table to reflect experimental ARM64 support and the planned status of RISC-V. There are no review comments to address, and I have no further feedback to provide.

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