Rollup merge of #145432 - Zalathar:target-machine, r=wesleywiser

cg_llvm: Small cleanups to `owned_target_machine`

This PR contains a few tiny cleanups to the `owned_target_machine` code.

Each individual commit should be fairly straightforward.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2443,7 +2443,7 @@ unsafe extern "C" {
         OutputObjFile: *const c_char,
         DebugInfoCompression: *const c_char,
         UseEmulatedTls: bool,
-        ArgsCstrBuff: *const c_char,
+        ArgsCstrBuff: *const c_uchar, // See "PTR_LEN_STR".
         ArgsCstrBuffLen: usize,
         UseWasmEH: bool,
     ) -> *mut TargetMachine;