Forbid calls to `asm!()` in safe contexts

[CohenArthur]

Needs #1568

[liamnaddell]

@CohenArthur This issue should be closed, it is no longer present in gccrs:

liam@gentoo /tmp $ cat inline_asm_nop.rs
#![feature(rustc_attrs)]

#[rustc_builtin_macro]
macro_rules! asm {
    () => {}
}

fn main() {
        asm!("nop");
        asm!("nop",);
}
liam@gentoo /tmp $ gccrs inline_asm_nop.rs
inline_asm_nop.rs:9:9: error: use of inline assembly is unsafe and requires unsafe function or block [E0133]
    9 |         asm!("nop");
      |         ^~~
inline_asm_nop.rs:10:9: error: use of inline assembly is unsafe and requires unsafe function or block [E0133]
   10 |         asm!("nop",);
      |         ^~~