PTW now does not require atomic memory operations, so take out the requirement (#767)

Bug fix in CSR which manifest itself when compiling a config with no extension

src/main/scala/rocket/CSR.scala

@@ -794,5 +794,5 @@ class CSRFile(perfEventSets: EventSets = new EventSets(Seq()))(implicit p: Param
     }
   }
   def formEPC(x: UInt) = ~(~x | Cat(!reg_misa('c'-'a'), UInt(1)))
-  def isaStringToMask(s: String) = s.map(x => 1 << (x - 'A')).reduce(_|_)
+  def isaStringToMask(s: String) = s.map(x => 1 << (x - 'A')).foldLeft(0)(_|_)
 }