rustbuild: Pass a threads flag that works to windows-gnu lld

MinGW driver for COFF LLD doesn't currently translate GNU-style `--threads=N` to native `/threads:N`, so we have to pass the option in its native form to avoid an error.

Also pass the `threads` flag to lld-link as well

src/bootstrap/test.rs

@@ -1110,6 +1110,19 @@ struct Compiletest {
     compare_mode: Option<&'static str>,
 }
 
+impl Compiletest {
+    fn add_lld_flags(builder: &Builder<'_>, target: TargetSelection, flags: &mut Vec<String>) {
+        if builder.config.use_lld {
+            if builder.is_fuse_ld_lld(target) {
+                flags.push("-Clink-arg=-fuse-ld=lld".to_string());
+            }
+
+            let threads = if target.contains("windows") { "/threads:1" } else { "--threads=1" };
+            flags.push(format!("-Clink-arg=-Wl,{}", threads));
+        }
+    }
+}
+
 impl Step for Compiletest {
     type Output = ();
 
@@ -1250,18 +1263,12 @@ note: if you're sure you want to do this, please open an issue as to why. In the
 
         let mut hostflags = flags.clone();
         hostflags.push(format!("-Lnative={}", builder.test_helpers_out(compiler.host).display()));
-        if builder.is_fuse_ld_lld(compiler.host) {
-            hostflags.push("-Clink-args=-fuse-ld=lld".to_string());
-            hostflags.push("-Clink-arg=-Wl,--threads=1".to_string());
-        }
+        Self::add_lld_flags(builder, compiler.host, &mut hostflags);
         cmd.arg("--host-rustcflags").arg(hostflags.join(" "));
 
         let mut targetflags = flags;
         targetflags.push(format!("-Lnative={}", builder.test_helpers_out(target).display()));
-        if builder.is_fuse_ld_lld(target) {
-            targetflags.push("-Clink-args=-fuse-ld=lld".to_string());
-            targetflags.push("-Clink-arg=-Wl,--threads=1".to_string());
-        }
+        Self::add_lld_flags(builder, target, &mut targetflags);
         cmd.arg("--target-rustcflags").arg(targetflags.join(" "));
 
         cmd.arg("--docck-python").arg(builder.python());