Merge #86

86: feat: Use new asm! instead of llvm_asm! r=Disasm a=duskmoon314

The new `asm` macro has been merged into `nightly` for some time, while `llvm_asm` will be gradually deprecated and will not enter `stable`. This PR replaces `llvm_asm` with `asm`, but there are still some minor issues to discuss.

My reference are:
- [asm feature](https://doc.rust-lang.org/unstable-book/library-features/asm.html)
- [arch/cortex-m: change a few llvm_asm!s to asm!s](https://github.com/tock/tock/pull/2092)
- [Convert all uses of llvm_asm! to asm!](https://github.com/rust-lang/stdarch/pull/1052)

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In the new `asm` feature, there are several options can set to optimize codes. [options](https://doc.rust-lang.org/unstable-book/library-features/asm.html#options-1

)

However, I'm not quite sure which options to set. (Sort of newbie to asm in rust)

Co-authored-by: Campbell He <hkp18@mails.tsinghua.edu.cn>