test(build-std): Add test for LTO (rust-lang#16277)

Add a test for rust-lang/rust#146133. `cargo
+nightly-2025-08-29 test --test build-std -- lto` can reproduce the
regression.
This is not a bug from Cargo, but it requires `-Zbuild-std` in most use
cases.
The test case is from rust-lang/rust#146109.
The point is that when rustc is invoked with -Clto=fat or -Clto=thin, it
should perform LTO with ALL bitcodes. However,
rust-lang/rust#145368 emits bitcodes for
compiler_builtins but excludes it from LTO participation.
As a result, the compiler_builtins bitcodes library is passed to the
linker, but the linkers, such as the GNU ld or older versions of LLD,
are unable to process bitcodes.

Author: weihanglo

tests/build-std/main.rs

@@ -187,6 +187,37 @@ fn basic() {
     assert_eq!(p.glob(deps_dir.join("*.dylib")).count(), 0);
 }
 
+#[cargo_test(build_std_real)]
+fn lto() {
+    // Checks that `-Zbuild-std` can work with `lto = "thin"`.
+    // This regression is from https://github.com/rust-lang/rust/issues/146109.
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2021"
+
+                [profile.dev]
+                lto = "thin"
+            "#,
+        )
+        .file(
+            "src/main.rs",
+            r#"
+            fn main() {}
+            "#,
+        )
+        .build();
+
+    p.cargo("build")
+        .build_std_arg("std")
+        .env("RUSTFLAGS", "-C linker-features=-lld")
+        .run();
+}
+
 #[cargo_test(build_std_real)]
 fn host_proc_macro() {
     let p = project()