Tidy gAborting.

Reduce scope to Runtime::Abort and short-cut recursive case earlier. gAborting
remains global to avoid two fatal errors in thread and the verifier.

Change-Id: Ibc893f891ffee9a763c65cde9507d99083d47b3f
diff --git a/runtime/base/mutex.cc b/runtime/base/mutex.cc
index aa2aefc..4957988 100644
--- a/runtime/base/mutex.cc
+++ b/runtime/base/mutex.cc
@@ -209,9 +209,7 @@
         }
       }
     }
-    if (gAborting == 0) {  // Avoid recursive aborts.
-      CHECK(!bad_mutexes_held);
-    }
+    CHECK(!bad_mutexes_held);
   }
 }