cpubits: add 64-bit only gating (#1400) #1643

Job	Run time
windows (x86_64-pc-windows-gnu, stable)	39s
build-only	23s
windows (x86_64-pc-windows-gnu, 1.85.0)	42s
cross (aarch64-unknown-linux-gnu, stable)	39s
macos (1.85.0)	32s
linux (x86_64-unknown-linux-gnu, 1.85.0)	15s
macos (stable)	19s
cross (aarch64-unknown-linux-gnu, 1.85.0)	40s
linux (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	34s
minimal-versions / minimal-versions	28s
linux (x86_64-unknown-linux-gnu, stable)	13s
linux (i686-unknown-linux-gnu, 1.85.0, sudo apt update && sudo apt install gcc-multilib)	34s
	5m 58s

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