Moving from llvm_asm! to asm! macro

[o8vm]

Although a new inline assembly syntax asm! was announced in Rust on 2020/06/08[1], KRaBs still uses llvm_am!.

[1] New inline assembly syntax available in nightly

According to the blog, the asm! macro should be safer and easier to use. This is expected to become the mainstream of inline assembly in the future, so KRaBs will have to transition to using asm! as well.
All llvm_asm! should be rewritten to the new asm! if it is rewritable.

The new inline assembly writing method is described in [2].

[2] rfcs/0000-inline-asm.md at inline-asm · Amanieu/rfcs