add fallback_version to register_coreutils_toolchains

[tx-chen]

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

Test plan

• Manual testing; please provide instructions so we can reproduce:

macos/x86_64, fix #838

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