[r+] Disallow a form of invalid asm! macro

[nagisa]

Fixes #21045

[rust-highfive]

r? @nick29581

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[pczarn]

This is worth labelling as a "breaking-change".
For me, concat! works as well, but some people may write code like this:

asm!("nop;"
     "nop;");

[nagisa]

It doesn’t work like that. It will currently only use the last string encountered as the assembly string. See #21045 for details.

EDIT: For all its worth

asm!("nop;
      nop;"); 

works just as fine

[pczarn]

Oh, it's certainly a bug.