bootstrap: open llvm_config as r+w

This previously failed on Windows because the `llvm_config` file was
open as read-only.

Author: jieyouxu

src/bootstrap/src/core/download.rs

@@ -1,7 +1,7 @@
 use std::{
     env,
     ffi::OsString,
-    fs::{self, File},
+    fs::{self, File, OpenOptions},
     io::{BufRead, BufReader, BufWriter, ErrorKind, Write},
     path::{Path, PathBuf},
     process::{Command, Stdio},
@@ -706,7 +706,7 @@ download-rustc = false
             let file_times = fs::FileTimes::new().set_accessed(now).set_modified(now);
 
             let llvm_config = llvm_root.join("bin").join(exe("llvm-config", self.build));
-            let llvm_config_file = t!(File::open(llvm_config));
+            let llvm_config_file = t!(OpenOptions::new().read(true).write(true).open(llvm_config));
 
             t!(llvm_config_file.set_times(file_times));