x86/rust: support MITIGATION_RETHUNK

The Rust compiler added support for `-Zfunction-return=thunk-extern` [1]
in 1.76.0 [2], i.e. the equivalent of `-mfunction-return=thunk-extern`.
Thus add support for `MITIGATION_RETHUNK`.

Without this, `objtool` would warn if enabled for Rust and already warns
under IBT builds, e.g.:

    samples/rust/rust_print.o: warning: objtool:
    _R...init+0xa5c: 'naked' return found in RETHUNK build

Link: https://github.com/rust-lang/rust/issues/116853 [1]
Link: https://github.com/rust-lang/rust/pull/116892 [2]
Reviewed-by: Gary Guo <[email protected]>
Tested-by: Alice Ryhl <[email protected]>
Tested-by: Benno Lossin <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Link: https://github.com/Rust-for-Linux/linux/issues/945
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

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