llvm: test switched-resume coroutine lowering

[cpunion]

Summary

• construct a canonical switched-resume coroutine through the public Go bindings
• verify both the explicit coroutine pass pipeline and the production default<O0> pipeline
• cover target-width frame intrinsics plus the LLVM 14/18/22 attribute and coro.end ABI differences

This is a test-only change; it does not add or change production bindings.

Testing

• go test ./...
• go vet ./...
• focused test with LLVM 14, 18, 19, 20, and the LLVM 22 forward-compatibility path
• GitHub CI covers the actual LLVM 21 matrix entry

[gemini-code-assist]

Code Review

This pull request adds a new test file coroutine_test.go to verify coroutine lowering pipelines in the LLVM Go bindings. Feedback was provided to initialize all LLVM targets instead of just the native target, ensuring the tests run robustly in cross-compilation environments where the default target triple might not match the host's native architecture.

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