cg_llvm: Add noreturn attribute to inline assembly marked as noreturn

[Fulgen301]

Inline assembly marked with options(noreturn) isn't actually annotated with LLVM's noreturn attribute, which can cause extra instructions to be generated:

pub fn foo() -> ! {
    unsafe { core::arch::asm!("int 0x29", options(noreturn)); }
}

currently generates

int 0x29
ud2

Other options are mapped to their respective LLVM attributes, so this seems like an oversight.

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