Revise bdw-gc.pc to specify atomic_ops library in Requires.private

[ivmai]

As suggested in PR #656 by @fundawang: It would be more correct to specify the internal dependency on libatomic_ops in Requires.private section of bdw-gc.pc.in rather than in Libs.private. But such change requires at least libatomic_ops v7.2, so this change is left for future (maybe waiting v8.5.0).