Fix std_detect not being an unstable crate #1898

Job	Run time
Check Style	15s
Automatic intrinsic verification	28s
Build std_detect	1m 2s
Build Documentation	1m 7s
Env Override	13s
Test (i686-unknown-linux-gnu)	1m 26s
Test (x86_64-unknown-linux-gnu)	1m 31s
Test (x86_64-unknown-linux-gnu-emulated)	1m 43s
Test (arm-unknown-linux-gnueabihf)	1m 14s
Test (armv7-unknown-linux-gnueabihf)	1m 45s
Test (aarch64-unknown-linux-gnu)	1m 47s
Test (riscv64gc-unknown-linux-gnu)	1m 20s
Test (powerpc64le-unknown-linux-gnu)	1m 44s
Test (s390x-unknown-linux-gnu)	1m 22s
Test (wasm32-wasi)	1m 22s
Test (i586-unknown-linux-gnu)	1m 49s
Test (nvptx64-nvidia-cuda)	26s
Test (thumbv6m-none-eabi)	12s
Test (thumbv7m-none-eabi)	13s
Test (thumbv7em-none-eabi)	14s
Test (thumbv7em-none-eabihf)	12s
Test (aarch64-apple-darwin)	1m 45s
Test (x86_64-pc-windows-msvc)	1m 38s
Test (i686-pc-windows-msvc)	1m 40s
Test (x86_64-apple-darwin, macos-11)	1m 42s
	28m 10s

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