[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.

[rustbot]

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[c410-f3r]

@rustbot modify labels: +T-libs-api -T-compiler

[davidtwco]

r? libs-api

[c410-f3r]

r? libs-api

[joshtriplett]

This looks great. Let's ship it.

[Noratrieb]

Please don't duplicate such large amounts of complicated code next time, it causes headaches when someone is trying to change it (which happened here). Instead it would have been nicer if you had reverted this back to use cfg_if or just plain cfg until the bootstrap compiler had the new cfg_match syntax.