Fix warnings about the native target-cpu

This fixes a regression from #53031 where specifying `-C target-cpu=native` is
printing a lot of warnings from LLVM about `native` being an unknown CPU. It
turns out that `native` is indeed an unknown CPU and we have to perform a
mapping to an actual CPU name, but this mapping is only performed in one
location rather than all locations we inform LLVM about the target CPU.

This commit centralizes the mapping of `native` to LLVM's value of the native
CPU, ensuring that all locations we inform LLVM about the `target-cpu` it's
never `native`.

Closes #53322

src/librustc_codegen_llvm/llvm_util.rs

@@ -17,6 +17,8 @@ use libc::c_int;
 use std::ffi::CString;
 use syntax::feature_gate::UnstableFeatures;
 
+use std::str;
+use std::slice;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Once;
 
@@ -262,3 +264,19 @@ pub(crate) fn print(req: PrintRequest, sess: &Session) {
         }
     }
 }
+
+pub fn target_cpu(sess: &Session) -> &str {
+    let name = match sess.opts.cg.target_cpu {
+        Some(ref s) => &**s,
+        None => &*sess.target.target.options.cpu
+    };
+    if name != "native" {
+        return name
+    }
+
+    unsafe {
+        let mut len = 0;
+        let ptr = llvm::LLVMRustGetHostCPUName(&mut len);
+        str::from_utf8(slice::from_raw_parts(ptr as *const u8, len)).unwrap()
+    }
+}