CL 400795, which uses the runtime/internal/atomic package in trace.go,
raced against CL 397014 removing that import. Re-add the import.
Change-Id: If847ec23f9a0fdff91dab07e93d9fb1b2efed85b
Reviewed-on: https://go-review.googlesource.com/c/go/+/403845
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