llvm_asm macros was changed to an asm

[humb1t]

New versions of Rust nightly fail to build old code:

/clear_on_drop-0.2.4/src/hide.rs:38:17
   |
38 |                 llvm_asm!("" : : "r" (ptr as *mut u8) : "memory");
   |                 ^^^^^^^^

error: cannot find macro `llvm_asm` in this scope

And rust-lang/rust#72965 which prevents fix of #23 was also resolved.
So I changed code back to asm usage with additional tweak - putting unused argument in comments.