fix benchmarks

The classpath is intended for type checking, not for compiler run-time.
Thus, they should be passed as compiler options, not JVM options.

It works before because the compiler makes bootstrap classpath
available for type checking. However, we can no longer use bootstrap
classpath after Java 9 support (#3138).

Author: liufengyun

bench/src/main/scala/Benchmarks.scala

@@ -34,11 +34,9 @@ object Bench {
     }
     storeCompileOptions(args2)
 
-    val libs = System.getProperty("BENCH_CLASS_PATH")
-
     val opts = new OptionsBuilder()
                .shouldFailOnError(true)
-               .jvmArgsPrepend(s"-classpath $libs", "-Xms2G", "-Xmx2G")
+               .jvmArgs("-Xms2G", "-Xmx2G")
                .mode(Mode.AverageTime)
                .timeUnit(TimeUnit.MILLISECONDS)
                .warmupIterations(warmup)
@@ -55,9 +53,13 @@ object Bench {
   def removeCompileOptions: Unit = new File(COMPILE_OPTS_FILE).delete()
 
   def storeCompileOptions(args: Array[String]): Unit = {
+    val libs = System.getProperty("BENCH_CLASS_PATH")
+
     val file = new File(COMPILE_OPTS_FILE)
     val bw = new BufferedWriter(new FileWriter(file))
-    bw.write(args.mkString("\n"))
+    bw.write(args.mkString("", "\n", "\n"))
+    bw.write("-classpath\n")
+    bw.write(libs)
     bw.close()
   }