avoid rustc_const_stable on internal function #2426

Job	Run time
Check Style	18s
Automatic intrinsic verification	27s
Test (i686-unknown-linux-gnu)	54s
Build std_detect	56s
Build Documentation	1m 13s
Test (x86_64-unknown-linux-gnu)	53s
Env Override (ubuntu-latest)	12s
Test (x86_64-unknown-linux-gnu-emulated)	53s
Env Override (windows-latest)	28s
Test (arm-unknown-linux-gnueabihf)	54s
Test (armv7-unknown-linux-gnueabihf)	56s
Test (aarch64-unknown-linux-gnu)	55s
Test (riscv64gc-unknown-linux-gnu)	54s
Test (powerpc64le-unknown-linux-gnu)	52s
Test (s390x-unknown-linux-gnu)	54s
Test (wasm32-wasip1)	39s
Test (i586-unknown-linux-gnu)	53s
Test (nvptx64-nvidia-cuda)	35s
Test (thumbv6m-none-eabi)	12s
Test (thumbv7m-none-eabi)	13s
Test (thumbv7em-none-eabi)	13s
Test (thumbv7em-none-eabihf)	14s
Test (loongarch64-unknown-linux-gnu)	54s
Test (x86_64-apple-darwin)	52s
Test (aarch64-apple-darwin)	37s
Test (x86_64-pc-windows-msvc)	48s
Test (i686-pc-windows-msvc)	48s
Test (aarch64-pc-windows-msvc)	47s
Test (x86_64-pc-windows-gnu)	47s
Test (aarch64-apple-ios-macabi, macos-latest, true)	55s
Test (x86_64-apple-ios-macabi, macos-13)	52s
success	1s
	21m 59s

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