Make dex2oat default to the architecture being built.

(cherry picked from commit 47874e73e50e03ebf5a2633ded7ea707ad29ddf6)

Change-Id: I3612a84989be930783e02e47f01012681ac153c7
diff --git a/src/dex2oat.cc b/src/dex2oat.cc
index 2770a96..c61bec1 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;