[codex] Fix single-arch Darwin universal binary packaging

[Saafo]

Reason

• When the iOS build config contains only one architecture, lipo -create was still invoked and produced an output that looked like a fat binary while containing a single architecture.
• Bazel Rust builds could not link against this ios-arm64/libogg.a artifact because the linker did not recognize it correctly.

Changes

• Check archs.len() in create_universal_binary.
• Copy the architecture-specific artifact directly into universal/lib when there is only one architecture.
• Only invoke lipo when there are two or more architectures.
• Verified with cargo fmt --check.