poly1305: rename `poly1305_force_soft` => `poly1305_backend="soft"` (… #515

Job	Run time
cross (powerpc-unknown-linux-gnu)	1m 16s
cross (aarch64-unknown-linux-gnu)	1m 10s
soft (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	32s
soft (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	30s
soft (x86_64-unknown-linux-gnu, stable)	15s
autodetect (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	34s
autodetect (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	36s
build (1.85.0, wasm32-unknown-unknown)	18s
build (stable, wasm32-unknown-unknown)	17s
soft (x86_64-unknown-linux-gnu, 1.85.0)	17s
build (stable, thumbv7em-none-eabi)	11s
avx2 (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	33s
autodetect (x86_64-unknown-linux-gnu, 1.85.0)	21s
autodetect (x86_64-unknown-linux-gnu, stable)	13s
build (1.85.0, thumbv7em-none-eabi)	19s
avx2 (x86_64-unknown-linux-gnu, stable)	18s
avx2 (x86_64-unknown-linux-gnu, 1.85.0)	21s
avx2 (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	37s
	8m 38s

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