The `asm!` macro has been renamed to `llvm_asm!`

[toothbrush7777777]

The asm! macro has been renamed to llvm_asm! (rust-lang/rust#68404) and using asm! was deprecated (rust-lang/rust#71007).
Using asm! instead of llvm_asm! is now a hard error in the last few versions of Rust nightly (see https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=9de7ebfc56af3593c526f4bd7bba26f9 for an example).
The new link in the unstable book is https://doc.rust-lang.org/nightly/unstable-book/library-features/llvm-asm.html.
Note also that the feature has been renamed from asm to llvm_asm.

I don't have the time to submit a pull request with the necessary changes to the code and documentation but this should be relatively easy to change.