cmov: use `black_box` in portable impl (#1255) #1939

Job	Run time
test (stable)	16s
build (stable, wasm32-unknown-unknown)	8s
test (1.85.0)	16s
build (1.85.0, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	14s
minimal-versions / minimal-versions	20s
build (1.85.0, thumbv7em-none-eabi)	19s
	1m 52s

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