correctly construct _ as Ident rather than Punctuation

[chrysn]

This fixes another bug like #1197: _ is no puctuation -- another case of dtolnay/proc-macro2#475, fall-out of dtolnay/proc-macro2#470.

Testing

This enables building RIOT sources again without --locked. I'm not entirely sure, but I think the offending assembly was

  __ASM ("rev %0, %1" : __CMSIS_GCC_OUT_REG (result) : __CMSIS_GCC_USE_REG (value) );

which gets turned into

    asm!(
        "rev {0}, {1}", lateout(reg) result, inlateout(reg) value => _,
        options(preserves_flags, pure, readonly)
    );

where the _ is the identifier (formerly punctuation) which c2rust trips over.

[chrysn]

Note that I did not put much thought into the span that is put in. Looks natural to use this one, could also have used span for less precision, but given this is all just used to generate a file (and not directly compiler input), I'm not sure it matters at all what is in there -- at least I wouldn't know how to trigger anything where it makes a difference and thus can be tested what the span in there means.

[fw-immunant]

LGTM. I don't think getting the span perfectly minimal is a big deal here.