Find codegen backends in more locations

* Search in the sysroot passed using `--sysroot` in addition to the
  default sysroot.
* Search for `librustc_codegen_$name.so` in addition to
  `librustc_codegen_$name-$release.so`.

compiler/rustc_driver/src/lib.rs

@@ -799,7 +799,7 @@ pub fn version(binary: &str, matches: &getopts::Matches) {
         println!("host: {}", config::host_triple());
         println!("release: {}", unw(util::release_str()));
         if cfg!(feature = "llvm") {
-            get_builtin_codegen_backend("llvm")().print_version();
+            get_builtin_codegen_backend(&None, "llvm")().print_version();
         }
     }
 }
@@ -1088,7 +1088,7 @@ pub fn handle_options(args: &[String]) -> Option<getopts::Matches> {
 
     if cg_flags.iter().any(|x| *x == "passes=list") {
         if cfg!(feature = "llvm") {
-            get_builtin_codegen_backend("llvm")().print_passes();
+            get_builtin_codegen_backend(&None, "llvm")().print_passes();
         }
         return None;
     }

compiler/rustc_interface/src/util.rs

@@ -241,7 +241,7 @@ pub fn get_codegen_backend(sopts: &config::Options) -> Box<dyn CodegenBackend> {
 
         let backend = match codegen_name {
             filename if filename.contains('.') => load_backend_from_dylib(filename.as_ref()),
-            codegen_name => get_builtin_codegen_backend(codegen_name),
+            codegen_name => get_builtin_codegen_backend(&sopts.maybe_sysroot, codegen_name),
         };
 
         unsafe {
@@ -366,15 +366,21 @@ fn sysroot_candidates() -> Vec<PathBuf> {
     }
 }
 
-pub fn get_builtin_codegen_backend(backend_name: &str) -> fn() -> Box<dyn CodegenBackend> {
+pub fn get_builtin_codegen_backend(
+    maybe_sysroot: &Option<PathBuf>,
+    backend_name: &str,
+) -> fn() -> Box<dyn CodegenBackend> {
     match backend_name {
         #[cfg(feature = "llvm")]
         "llvm" => rustc_codegen_llvm::LlvmCodegenBackend::new,
-        _ => get_codegen_sysroot(backend_name),
+        _ => get_codegen_sysroot(maybe_sysroot, backend_name),
     }
 }
 
-pub fn get_codegen_sysroot(backend_name: &str) -> fn() -> Box<dyn CodegenBackend> {
+pub fn get_codegen_sysroot(
+    maybe_sysroot: &Option<PathBuf>,
+    backend_name: &str,
+) -> fn() -> Box<dyn CodegenBackend> {
     // For now we only allow this function to be called once as it'll dlopen a
     // few things, which seems to work best if we only do that once. In
     // general this assertion never trips due to the once guard in `get_codegen_backend`,
@@ -389,8 +395,9 @@ pub fn get_codegen_sysroot(backend_name: &str) -> fn() -> Box<dyn CodegenBackend
     let target = session::config::host_triple();
     let sysroot_candidates = sysroot_candidates();
 
-    let sysroot = sysroot_candidates
+    let sysroot = maybe_sysroot
         .iter()
+        .chain(sysroot_candidates.iter())
         .map(|sysroot| {
             let libdir = filesearch::relative_target_lib_path(&sysroot, &target);
             sysroot.join(libdir).with_file_name("codegen-backends")
@@ -426,8 +433,10 @@ pub fn get_codegen_sysroot(backend_name: &str) -> fn() -> Box<dyn CodegenBackend
 
     let mut file: Option<PathBuf> = None;
 
-    let expected_name =
-        format!("rustc_codegen_{}-{}", backend_name, release_str().expect("CFG_RELEASE"));
+    let expected_names = &[
+        format!("rustc_codegen_{}-{}", backend_name, release_str().expect("CFG_RELEASE")),
+        format!("rustc_codegen_{}", backend_name),
+    ];
     for entry in d.filter_map(|e| e.ok()) {
         let path = entry.path();
         let filename = match path.file_name().and_then(|s| s.to_str()) {
@@ -438,7 +447,7 @@ pub fn get_codegen_sysroot(backend_name: &str) -> fn() -> Box<dyn CodegenBackend
             continue;
         }
         let name = &filename[DLL_PREFIX.len()..filename.len() - DLL_SUFFIX.len()];
-        if name != expected_name {
+        if !expected_names.iter().any(|expected| expected == name) {
             continue;
         }
         if let Some(ref prev) = file {