Port over to new asm syntax.

[recmo]

On 2020-08-17 @recmo wrote in a03e7b2 “Merge branch 'master' into remco/optim/asm”:

Port over to new asm syntax.

// Some routines have assembly optimized versions available for some
// architectures
// TODO: No asm on stable
// See <https://github.com/rust-lang/rust/issues/29722>
#![cfg_attr(feature = "asm", feature(asm))]
// TODO: Port over to new asm syntax.
#![cfg_attr(feature = "asm", feature(llvm_asm))]

mod additive;
pub(crate) mod algorithms;
mod arch;

From algebra/u256/src/lib.rs:54