Fails to build on nightly after const fn is stable

[kornelski]

error[E0557]: feature has been removed
Error:   --> /Users/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/extprim-1.7.1/src/lib.rs:58:81
   |
58 | #![cfg_attr(extprim_channel="unstable", feature(llvm_asm, test, specialization, const_fn))]
   |                                                                                 ^^^^^^^^ feature has been removed
   |
   = note: split into finer-grained feature gates

rustc 1.54.0-nightly (21e92b973 2021-05-12)