fix reflect MakeFunc goroutine liveness

[cpunion]

Summary

• Keep reflect.MakeFunc libffi closure state (closure, Signature, callback data) reachable through the returned function environment.
• Put goroutine startup records in scanned uncollectable runtime memory until the entry call returns, so reflect-generated function args stay visible while the new thread starts.
• Add stable test/go coverage for reflect-generated functions launched as goroutines under a racing runtime.GC loop.
• Remove only the now-passing fixedbugs/issue25897a.go xfails.

Dependency / merge order

• Depends on reflect: fix type metadata and MakeFunc results #1916 merging first.
• This PR is a draft goroutine/liveness follow-up and should be rebased after reflect: fix type metadata and MakeFunc results #1916 lands.
• The expected conflict is in runtime/internal/lib/reflect/makefunc.go: preserve reflect: fix type metadata and MakeFunc results #1916's MakeFunc result validation/storage changes and combine them with this PR's closure/signature lifetime rooting changes.

Notes

• Full GOROOT CI is slow/disabled locally; I ran only the requested targeted GOROOT case.
• Normal PR CI should run from the fork branch. I did not push anything to xgo-dev/llgo.

Focused tests

• Reproduced before fix with xfail disabled:
  go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot /opt/homebrew/Cellar/go@1.24/1.24.11/libexec -dirs fixedbugs -case '^fixedbugs/issue25897a\\.go$' -xfail /tmp/llgo-empty-xfail.yaml
• go test ./test/go -run '^TestReflectMakeFuncGoroutineStartup$' -count=1
• go test ./test/go -count=1
• go run ./cmd/llgo test -a -run '^TestReflectMakeFuncGoroutineStartup$' ./test/go
• go test ./ssa -run '^TestGoClosureStartupUsesGCManagedMemory$' -count=1
• go test ./ssa -run 'TestGoClosureStartupUsesGCManagedMemory|TestMakeClosureWithCtx|TestClosure' -count=1
• (cd runtime && go test ./... -count=1)
• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot /opt/homebrew/Cellar/go@1.24/1.24.11/libexec -dirs fixedbugs -case '^fixedbugs/issue25897a\\.go$' -xfail /tmp/llgo-empty-xfail.yaml -llgo /tmp/llgo-dev
• go test ./test/goroot -count=1 -run TestGoRootRunCases -args -goroot /opt/homebrew/Cellar/go@1.24/1.24.11/libexec -dirs fixedbugs -case '^fixedbugs/issue25897a\\.go$' -llgo /tmp/llgo-dev

[cpunion]

修复进展（head 1d04791cb280439c315bd568b2b20896152a3eb3）：

根因：失败 job test (ubuntu-latest, 19) 的完整日志显示 go test -timeout 30m ./... 里 cl 和 ssa 的 TestRunAndTestFromTestgo/goroutine、selects FileCheck 失败。分支实现已把 goroutine startup record 从 AllocU 改为 AllocRoot，并在 routine 返回前 FreeRoot，但 cl/_testgo/goroutine 和 cl/_testgo/selects 的旧 IR 期望仍匹配 AllocU。

修复：更新 cl/_testgo/goroutine/in.go 和 cl/_testgo/selects/in.go 的 CHECK 期望，匹配 AllocRoot，并在 goroutine routine 检查 FreeRoot。功能覆盖保留在 test/go/reflect_makefunc_goroutine_test.go，覆盖 reflect.MakeFunc 生成的函数在 goroutine + 强制 GC 下的 liveness。

本地测试：

• go test ./cl -run 'TestRunAndTestFromTestgo/(goroutine|selects)$' -count=1 ✅
• go test ./ssa -run 'TestFromTestgo/(goroutine|selects)$|TestGoClosureStartupUsesGCManagedMemory' -count=1 ✅
• go test ./test/go -count=1 ✅
• go test ./cl -run TestRunAndTestFromTestgo -count=1 ✅
• go test ./ssa -run 'TestFromTestgo|TestGoClosureStartupUsesGCManagedMemory' -count=1 ✅
• go test ./ssa -count=1 ✅

CI 状态：新 head CI 已触发，目前 checks 仍在 queued/in_progress，我会继续监控并按失败继续修。

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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