Make dex2oat default to the architecture being built.

Change-Id: I1df259da7f5486146b864f3597cffdd4ebbe6780
diff --git a/src/dex2oat.cc b/src/dex2oat.cc
index 0fa9cd7..32883d3 100644
--- a/src/dex2oat.cc
+++ b/src/dex2oat.cc
@@ -477,7 +477,15 @@
   std::vector<const char*> runtime_args;
   int thread_count = 2;
   bool support_debugging = false;
+#if defined(__arm__)
   InstructionSet instruction_set = kThumb2;
+#elif defined(__i386__)
+  InstructionSet instruction_set = kX86;
+#elif defined(__mips__)
+  InstructionSet instruction_set = kMips;
+#else
+#error "Unsupported architecture"
+#endif
   bool dump_stats = kIsDebugBuild;
   bool dump_timings = kIsDebugBuild;