Replace `llvm_asm!` with `asm!`

Since rust removed `llvm_asm!`: https://github.com/rust-lang/rust/commit/a34c0797528172ede89480e3033f7a5e71ea4735

https://github.com/rust-lang/packed_simd/blob/b47552659401299a2023e7f408316bfe2dd4476d/src/lib.rs#L227