Ruby

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