Set default dex2oat instruction set for x86-64 and ARM64.

Change-Id: I43062dfadb8a4f51304fd06f0105d6af81cf0331
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index cc78816..327d3fb 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -738,8 +738,12 @@
 
 #if defined(__arm__)
   InstructionSet instruction_set = kThumb2;
+#elif defined(__aarch64__)
+  InstructionSet instruction_set = kArm64;
 #elif defined(__i386__)
   InstructionSet instruction_set = kX86;
+#elif defined(__x86_64__)
+  InstructionSet instruction_set = kX86_64;
 #elif defined(__mips__)
   InstructionSet instruction_set = kMips;
 #else