Use sysroot instead of CFG_PREFIX for the rpath

CFG_PREFIX is incorrect for rustup installed rustc versions. It also
causes unnecessary recompilation when changing the install prefix.

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1567,13 +1567,12 @@ fn add_rpath_args(
 ) {
     // FIXME (#2397): At some point we want to rpath our guesses as to
     // where extern libraries might live, based on the
-    // addl_lib_search_paths
+    // add_lib_search_paths
     if sess.opts.cg.rpath {
         let target_triple = sess.opts.target_triple.triple();
         let mut get_install_prefix_lib_path = || {
-            let install_prefix = option_env!("CFG_PREFIX").expect("CFG_PREFIX");
             let tlib = rustc_target::target_rustlib_path(&sess.sysroot, target_triple).join("lib");
-            let mut path = PathBuf::from(install_prefix);
+            let mut path = PathBuf::from(&sess.sysroot);
             path.push(&tlib);
 
             path

src/bootstrap/compile.rs

@@ -629,8 +629,7 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
     cargo
         .env("CFG_RELEASE", builder.rust_release())
         .env("CFG_RELEASE_CHANNEL", &builder.config.channel)
-        .env("CFG_VERSION", builder.rust_version())
-        .env("CFG_PREFIX", builder.config.prefix.clone().unwrap_or_default());
+        .env("CFG_VERSION", builder.rust_version());
 
     let libdir_relative = builder.config.libdir_relative().unwrap_or_else(|| Path::new("lib"));
     cargo.env("CFG_LIBDIR_RELATIVE", libdir_relative);