cmov: use `black_box` in portable impl (#1255) #1919

Job	Run time
test (1.85.0)	12s
test (stable)	35s
build (stable, wasm32-unknown-unknown)	12s
build (1.85.0, wasm32-unknown-unknown)	18s
build (stable, thumbv7em-none-eabi)	24s
build (1.85.0, thumbv7em-none-eabi)	15s
minimal-versions / minimal-versions	28s
	2m 24s

Provide feedback