runtime: report BDWGC cycles in MemStats

[cpunion]

Summary

• Initialize BDWGC for the runtime package and populate runtime.MemStats from BDWGC heap/cycle counters.
• Add test/go coverage for a goroutine plus runtime.GC during init.
• Remove only the locally verified darwin/arm64 xfail for GOROOT/test/init1.go.

Tests

• go test ./test/go -count=1 -run TestRuntimeGCStatsDuringInit
• go test ./test/goroot -count=1 -run TestGoRoot -args -goroot "/opt/homebrew/Cellar/go@1.24/1.24.11/libexec" -dirs . -case '^init1.go$' -xfail /tmp/llgo-empty-xfail.yaml
• (cd runtime && go test ./internal/clite/bdwgc -count=1)
• (cd runtime && go test ./internal/lib/runtime -count=1)
• go test ./test/go -count=1
• go test ./test/goroot -count=1 -run TestGoRoot -args -goroot "/opt/homebrew/Cellar/go@1.24/1.24.11/libexec" -dirs . -case '^init1.go$'

Local GOROOT: /opt/homebrew/Cellar/go@1.24/1.24.11/libexec, go1.24.11 darwin/arm64.

CI note

GOROOT CI is slow/disabled, so this PR relies on normal PR CI from the cpunion fork branch. I did not push to xgo-dev/llgo. If manual GOROOT workflow_dispatch cannot resolve fork refs, that remains a workflow blocker to document rather than a reason to push directly to xgo-dev.

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