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Use portable-atomic when AtomicU64 is not available. #602

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Job	Run time
msrv	28s
audit	19s
coverage	50s
lint	25s
test (ubuntu-latest, stable)	1m 26s
test (ubuntu-latest, beta)	1m 35s
test (macos-latest, stable)	1m 26s
test (windows-latest, stable)	4m 7s
	10m 36s