bootstrap: open llvm_config as r+w

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

src/bootstrap/src/core/download.rs

@@ -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!(File::options().write(true).open(llvm_config));
 
  t!(llvm_config_file.set_times(file_times));