cmov: fix portable `cmovnz` for `thumbv6m-none-eabi` (#1332) #157

Job	Run time
test-careful	1m 27s
test-miri (s390x-unknown-linux-gnu)	1m 1s
test (1.85.0)	1m 6s
test-miri (x86_64-unknown-linux-gnu)	49s
build (1.85.0, thumbv7em-none-eabi)	18s
build (stable, wasm32-unknown-unknown)	12s
build (stable, thumbv7em-none-eabi)	9s
build (1.85.0, wasm32-unknown-unknown)	15s
test (stable)	1m 10s
minimal-versions / minimal-versions	59s
	7m 26s

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