Remove unnecessary stability attributes for const fn

[jethrogb]

In 8d6edc9 various stability attributes were added to functions that are not public. This was required to satisfy the min_const_fn checker. The lines in question all contain FIXME: min_const_fn. @oli-obk volunteered to take a look.

You should be able to test this build with a somewhat-recent nightly and

cargo +nightly check --target x86_64-fortanix-unknown-sgx --manifest-path src/libstd/Cargo.toml