[cfg_match] Adjust syntax

[c410-f3r]

A year has passed since the creation of #115585 and the feature, as expected, is not moving forward. Let's change that.

This PR proposes changing the arm's syntax from cfg(SOME_CONDITION) => { ... } to SOME_CODITION => {}.

match_cfg! {
   unix => {
        fn foo() { /* unix specific functionality */ }
    }
    target_pointer_width = "32" => {
        fn foo() { /* non-unix, 32-bit functionality */ }
    }
    _ => {
        fn foo() { /* fallback implementation */ }
    }
}

Why? Because after several manual migrations in #116342 it became clear, at least for me, that cfg prefixes are unnecessary, verbose and redundant.

Again, everything is just a proposal to move things forward. If the shown syntax isn't ideal, feel free to close this PR or suggest other alternatives.