Auto merge of #136586 - Kobzol:lto-rustdoc-fix-stage-1, r=<try>

Only apply LTO to rustdoc at stage 2

It doesn't make much sense at stage 1, and it was broken anyway. This was implemented in #135832. The issue with LTO and stage 1 rustdoc was reported [here](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/x.20test.20with.20lto.20.3D.20.22thin.22.20fails.20to.20build.20rustdoc.3F).

r? `@onur-ozkan`

Author: bors

src/bootstrap/src/core/build_steps/tool.rs

@@ -660,14 +660,19 @@ impl Step for Rustdoc {
         );
 
         // rustdoc is performance sensitive, so apply LTO to it.
-        let lto = match builder.config.rust_lto {
-            RustcLto::Off => Some("off"),
-            RustcLto::Thin => Some("thin"),
-            RustcLto::Fat => Some("fat"),
-            RustcLto::ThinLocal => None,
-        };
-        if let Some(lto) = lto {
-            cargo.env(cargo_profile_var("LTO", &builder.config), lto);
+        // The compiler shared libraries (librustc_driver.so) don't use LTO on stages
+        // lower than stage 1, as it's not worth it.
+        // So we also need to only do this at stage 2+, to avoid missing LTO bitcode errors.
+        if build_compiler.stage != 0 {
+            let lto = match builder.config.rust_lto {
+                RustcLto::Off => Some("off"),
+                RustcLto::Thin => Some("thin"),
+                RustcLto::Fat => Some("fat"),
+                RustcLto::ThinLocal => None,
+            };
+            if let Some(lto) = lto {
+                cargo.env(cargo_profile_var("LTO", &builder.config), lto);
+            }
         }
 
         let _guard = builder.msg_tool(