cmov: extract portable `testeq` and `testne` functions (#1340) #2077

Job	Run time
test (1.85.0)	16s
build (1.85.0, thumbv7em-none-eabi)	18s
build (stable, thumbv7em-none-eabi)	38s
test (stable)	32s
build (stable, wasm32-unknown-unknown)	22s
minimal-versions / minimal-versions	27s
build (1.85.0, wasm32-unknown-unknown)	17s
	2m 50s

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