YJIT: Improve checking message for rustc version (#6693)

Preivously we didn't have a "checking ...." line for this check and when
rustc was too old, we would dump the error message to the console like:

    checking for rustc... rustc
    error: there is no argument named `x`
     --> <anon>:1:33
      |
    1 | fn main() { let x = 1; format!("{x}"); }
      |                                 ^^^

    error: aborting due to previous error

`configure` checks usually don't do this and this might be confusing.

With this commit it now says something like:

    checking whether rustc is new enough for YJIT... no

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