Driver: correct the backend option spelling

The backend option does not have an '-enable' prefix.

llvm-svn: 211177
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index e49267a..2f86621 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -3479,7 +3479,7 @@
                                  options::OPT_mno_long_calls)) {
       if (A->getOption().matches(options::OPT_mlong_calls)) {
         CmdArgs.push_back("-backend-option");
-        CmdArgs.push_back("-enable-arm-long-calls");
+        CmdArgs.push_back("-arm-long-calls");
       }
     }
   }