reflect: handle empty fields in FFI struct calls

[cpunion]

Summary

• Skip zero-sized struct fields when building reflect libffi struct argument types, while preserving explicit gaps before later non-zero fields.
• Add test/go coverage for reflect.Value.Call with a struct containing an empty trailing field followed by a slice argument.
• Remove the darwin/arm64 GOROOT xfail for fixedbugs/issue26335.go after targeted local pass.

Focused tests

• go test ./test/goroot -run TestGoRootRunCases -count=1 -args -goroot "$(go env GOROOT)" -llgo /tmp/llgo-reflectcall -dirs fixedbugs -case "^fixedbugs/issue26335\.go$" -xfail /tmp/llgo-empty-xfail.yaml -run-timeout 30s -build-timeout 2m
• go test ./test/goroot -run TestGoRootRunCases -count=1 -args -goroot "$(go env GOROOT)" -llgo /tmp/llgo-reflectcall -dirs fixedbugs -case "^fixedbugs/issue26335\.go$" -run-timeout 30s -build-timeout 2m
• /tmp/llgo-reflectcall test -run TestReflectCallStructWithEmptyField ./test/go
• go test ./test/go -run TestReflectCallStructWithEmptyField -count=1
• (cd runtime && go test ./internal/lib/reflect ./internal/ffi -count=1)

GOROOT CI

The full GOROOT workflow is workflow_dispatch-only and slow/disabled for normal PR CI. I kept the run targeted locally with the current machine GOROOT (go env GOROOT = /opt/homebrew/Cellar/go@1.24/1.24.11/libexec). If manual GOROOT dispatch cannot resolve fork refs, this PR should rely on normal PR CI from the cpunion/llgo fork branch and keep that blocker documented here rather than pushing any branch to xgo-dev/llgo.

Notes

I only removed the darwin/arm64 xfail entry verified locally. Linux version-specific xfails for the same upstream case remain until they can be executed on Linux.

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