Directly add extension instead of using Path::with_extension

`Path::with_extension` has a nice footgun when the original path doesn't
contain an extension: Anything after the last dot gets removed.

compiler/rustc_codegen_llvm/src/back/archive.rs

@@ -129,8 +129,7 @@ impl ArchiveBuilderBuilder for LlvmArchiveBuilderBuilder {
         let name_suffix = if is_direct_dependency { "_imports" } else { "_imports_indirect" };
         let output_path = {
             let mut output_path: PathBuf = tmpdir.to_path_buf();
-            output_path.push(format!("{lib_name}{name_suffix}"));
-            output_path.with_extension("lib")
+            output_path.push(format!("{lib_name}{name_suffix}.lib"));
         };
 
         let target = &sess.target;
@@ -157,7 +156,7 @@ impl ArchiveBuilderBuilder for LlvmArchiveBuilderBuilder {
             // functions. Therefore, use binutils to create the import library instead,
             // by writing a .DEF file to the temp dir and calling binutils's dlltool.
             let def_file_path =
-                tmpdir.join(format!("{lib_name}{name_suffix}")).with_extension("def");
+                tmpdir.join(format!("{lib_name}{name_suffix}.def"));
 
             let def_file_content = format!(
                 "EXPORTS\n{}",