runtime: revert entry point on freebsd/arm64

[rdnn]

CL 706175 unified some arm64 entry points. However, the entry point for
freebsd/arm64 is incorrect and builds now fail. This change reverts the
freebsd/arm64 entry point and adds additional context for the future.

Fixes #76958

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This PR is being closed because golang.org/cl/731980 has been merged.