title: clang target option for riscv64 doesn't work
body: <p>I'm trying to use clang to make IR for riscv64.</p>
  
  <p>When I use llc --version, it shows many targets including riscv64.</p>
  
  <p>But when I use the following command:</p>
  
  <pre><code>clang -target riscv64 hello.c
  clang -target riscv64-unknown-linux hello.c
  </code></pre>
  
  <p>It shows <strong>clang-4.0: error: unknown target triple 'riscv64-unknown-linux', please use -triple or -arch</strong></p>
  
  <p>I'm not using ucb-bar's llvm for riscv. I'm using riscv from upstream of llvm.org.</p>
  
  <p>Did I do something wrong or do I have to do something before building llvm?</p>