implement feature(c_variadic_naked_functions)

[folkertdev]

tracking issue: #148767

#t-lang > C-variadic naked functions

This feature allows naked c-variadic function definitions with any ABI that is supported for foreign c-variadic functions.

#![feature(c_variadic, c_variadic_naked_functions)]

#[unsafe(naked)]
unsafe extern "win64" fn variadic_win64(_: u32, _: ...) -> u32 {
    core::arch::naked_asm!(
        r#"
        push    rax
        mov     qword ptr [rsp + 40], r9
        mov     qword ptr [rsp + 24], rdx
        mov     qword ptr [rsp + 32], r8
        lea     rax, [rsp + 40]
        mov     qword ptr [rsp], rax
        lea     eax, [rdx + rcx]
        add     eax, r8d
        pop     rcx
        ret
    "#,
    )
}

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