ART: Use _exit in dex2oat

That's what we really wanted to call. Seems to save about 50ms when
compiling something small-ish (out of .8s wall-clock time).

Test: m test-art-host
Change-Id: I9a20451adeaf63f535a7e42b46b4db78ee1d2bea
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index b4ea20b..81566c4 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -2931,7 +2931,7 @@
   // time (bug 10645725) unless we're a debug build or running on valgrind. Note: The Dex2Oat class
   // should not destruct the runtime in this case.
   if (!art::kIsDebugBuild && (RUNNING_ON_MEMORY_TOOL == 0)) {
-    exit(result);
+    _exit(result);
   }
   return result;
 }