std: Tweak expansion of thread-local const

This commit tweaks the expansion of `thread_local!` when combined with a
`const { ... }` value to help ensure that the rules which apply to
`const { ... }` blocks will be the same as when they're stabilized.
Previously with this invocation:

    thread_local!(static NAME: Type = const { init_expr });

this would generate (on supporting platforms):

    #[thread_local]
    static NAME: Type = init_expr;

instead the macro now expands to:

    const INIT_EXPR: Type = init_expr;
    #[thread_local]
    static NAME: Type = INIT_EXPR;

with the hope that because `init_expr` is defined as a `const` item then
it's not accidentally allowing more behavior than if it were put into a
`static`. For example on the stabilization issue [this example][ex] now
gives the same error both ways.

[ex]: https://github.com/rust-lang/rust/issues/84223#issuecomment-953384298

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