avoid rustc_const_stable on internal function #2427

Job	Run time
Check Style	14s
Automatic intrinsic verification	31s
Build Documentation	17s
Build std_detect	1m 0s
Env Override (ubuntu-latest)	14s
Env Override (windows-latest)	26s
Test (i686-unknown-linux-gnu)	14s
Test (x86_64-unknown-linux-gnu)	27s
Test (x86_64-unknown-linux-gnu-emulated)	27s
Test (arm-unknown-linux-gnueabihf)	28s
Test (armv7-unknown-linux-gnueabihf)	28s
Test (aarch64-unknown-linux-gnu)	27s
Test (riscv64gc-unknown-linux-gnu)	27s
Test (powerpc64le-unknown-linux-gnu)	28s
Test (s390x-unknown-linux-gnu)	15s
Test (wasm32-wasip1)	14s
Test (i586-unknown-linux-gnu)	28s
Test (nvptx64-nvidia-cuda)	27s
Test (thumbv6m-none-eabi)	13s
Test (thumbv7m-none-eabi)	13s
Test (thumbv7em-none-eabi)	16s
Test (thumbv7em-none-eabihf)	15s
Test (loongarch64-unknown-linux-gnu)	16s
Test (x86_64-apple-darwin)	17s
Test (aarch64-apple-darwin)	16s
Test (x86_64-pc-windows-msvc)	12s
Test (i686-pc-windows-msvc)	15s
Test (aarch64-pc-windows-msvc)	12s
Test (x86_64-pc-windows-gnu)	13s
Test (aarch64-apple-ios-macabi, macos-latest, true)	15s
Test (x86_64-apple-ios-macabi, macos-13)	18s
success	1s
	10m 44s

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