win: Copy libcc/compiler-rt to the sysroot and use it

Instead of hardcoding -lgcc. This allows to use compiler-rt when
compiling with clang.
Since -nodefaultlibs is used, compiler-rt isn't added by default,
causing various linking failures.

Author: chouquette

src/bootstrap/compile.rs

@@ -329,6 +329,8 @@ impl Step for StartupObjects {
                                     obj);
             builder.copy(&src, &sysroot_dir.join(obj));
         }
+        let libgcc = compiler_libgcc(builder, builder.cc(target));
+        builder.copy(&libgcc, &sysroot_dir.join("libgcc.a"));
     }
 }
 
@@ -890,6 +892,18 @@ pub fn compiler_file(
     PathBuf::from(out.trim())
 }
 
+/// Get the path to libgcc or compiler-rt
+fn compiler_libgcc(
+    builder: &Builder<'_>,
+    compiler: &Path
+) -> PathBuf {
+    let mut cmd = Command::new(compiler);
+    cmd.arg("-print-libgcc-file-name");
+    builder.info(&format!("compiler cmd: {:?}", &cmd));
+    let out = output(&mut cmd);
+    PathBuf::from(out.trim())
+}
+
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub struct Sysroot {
     pub compiler: Compiler,

src/librustc_target/spec/windows_base.rs

@@ -46,7 +46,6 @@ pub fn opts() -> TargetOptions {
     late_link_args.insert(LinkerFlavor::Gcc, vec![
         "-lmingwex".to_string(),
         "-lmingw32".to_string(),
-        "-lgcc".to_string(), // alas, mingw* libraries above depend on libgcc
         "-lmsvcrt".to_string(),
         // mingw's msvcrt is a weird hybrid import library and static library.
         // And it seems that the linker fails to use import symbols from msvcrt
@@ -87,7 +86,8 @@ pub fn opts() -> TargetOptions {
         ],
         late_link_args,
         post_link_objects: vec![
-            "rsend.o".to_string()
+            "rsend.o".to_string(),
+            "libgcc.a".to_string(), // libgcc or compiler-rt
         ],
         custom_unwind_resume: true,
         abi_return_struct_as_int: true,