Rollup merge of #73364 - joshtriplett:inline-asm, r=Amanieu

asm: Allow multiple template string arguments; interpret them as newline-separated

Allow the `asm!` macro to accept a series of template arguments, and interpret them as if they were concatenated with a '\n' between them. This allows writing an `asm!` where each line of assembly appears in a separate template string argument.

This syntax makes it possible for rustfmt to reliably format and indent each line of assembly, without risking changes to the inside of a template string. It also avoids the complexity of having the user carefully format and indent a multi-line string (including where to put the surrounding quotes), and avoids the extra indentation and lines of a call to `concat!`.

For example, rewriting the second example from the [blog post on the new inline assembly syntax](https://blog.rust-lang.org/inside-rust/2020/06/08/new-inline-asm.html) using multiple template strings:

```rust

fn main() {
    let mut bits = [0u8; 64];
    for value in 0..=1024u64 {
        let popcnt;
        unsafe {
            asm!(
                "    popcnt {popcnt}, {v}",
                "2:",
                "    blsi rax, {v}",
                "    jz 1f",
                "    xor {v}, rax",
                "    tzcnt rax, rax",
                "    stosb",
                "    jmp 2b",
                "1:",
                v = inout(reg) value => _,
                popcnt = out(reg) popcnt,
                out("rax") _, // scratch
                inout("rdi") bits.as_mut_ptr() => _,
            );
        }
        println!("bits of {}: {:?}", value, &bits[0..popcnt]);
    }
}
```

Note that all the template strings must appear before all other arguments; you cannot, for instance, provide a series of template strings intermixed with the corresponding operands.

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