p521: fiat-constify update #1968

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 42s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	2m 28s
build (1.65.0, wasm32-unknown-unknown)	1m 38s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 38s
build (stable, thumbv7em-none-eabi)	37s
doc	23s
cross (aarch64-unknown-linux-gnu, 1.65.0)	2m 18s
build (stable, wasm32-unknown-unknown)	39s
cross (aarch64-unknown-linux-gnu, stable)	1m 20s
cross (powerpc-unknown-linux-gnu, 1.65.0)	2m 29s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	2m 47s
cross (powerpc-unknown-linux-gnu, stable)	1m 20s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 44s
test (x86_64-unknown-linux-gnu, 1.65.0)	2m 31s
test (x86_64-unknown-linux-gnu, stable)	1m 28s
	25m 2s

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