Move to new inline asm syntax

[roblabla]

We should move to the new inline asm syntax instead of using llvm_asm, as it has a clear path to stabilization and is, in general, a lot less buggy.

Doing so will also make a lot of our code less UB "by default", as rust's inline asm defaults to making the code paranoid (e.g. it defaults to volatile, memory, etc... and has flag to disable them, instead of llvm_asm where by default the compiler assumes it can do a lot of opts that you have to manually disable).