Compile `UNADJUSTED` ABI extern functions

[Polygonalr]

Continuation of #4791, we do not support compilation of LLVM built-ins yet, e.g:

#[allow(improper_ctypes)]
extern "unadjusted" {
    #[link_name = "llvm.x86.rdrand.16"]
    fn x86_rdrand16_step() -> (u16, i32);
    #[link_name = "llvm.x86.rdrand.32"]
    fn x86_rdrand32_step() -> (u32, i32);
    #[link_name = "llvm.x86.rdseed.16"]
    fn x86_rdseed16_step() -> (u16, i32);
    #[link_name = "llvm.x86.rdseed.32"]
    fn x86_rdseed32_step() -> (u32, i32);
}

I do not have permissions to assign myself the issue, but work is already in progress in #4809.