Add formatting support for the `asm!` macro

Given a snippet like this

unsafe fn foobar() {
    core::arch::asm!("{}", 
        const X)
}

I'd expect that to format to

unsafe fn foobar() {
    core::arch::asm!("{}", const X)
}

but instead nothing happens. Given that these operands are quite new, I suspect this is an oversight.

I'd happily look into fixing this, but I'm not 100% clear on whether the formatting behavior for existing code can be change? Though here I'd clearly consider it a bug.

Reproduce the issue by running rustfmt on the provided unsafe function and compare the output with the expected formatting for the asm! operands. Trace how rustfmt handles asm! arguments, then add regression coverage for this snippet; done means the const X operand is kept on the same line as the format string.