Generic JNI implementation for x86_64

Starting implementation for generic JNI on x86_64. Frames are of
large static size (>4K) right now, should be compacted later. Passes
the whole of jni_compiler_test.

Change-Id: I88ac3e13a534afe7568d62a1ef97cb766e8260e4
diff --git a/runtime/stack.h b/runtime/stack.h
index 6a62922..f840f67 100644
--- a/runtime/stack.h
+++ b/runtime/stack.h
@@ -532,7 +532,7 @@
     DCHECK(GetMethod() != nullptr);
     byte* save_addr =
         reinterpret_cast<byte*>(cur_quick_frame_) + frame_size - ((num + 1) * kPointerSize);
-#if defined(__i386__)
+#if defined(__i386__) || defined(__x86_64__)
     save_addr -= kPointerSize;  // account for return address
 #endif
     return reinterpret_cast<uintptr_t*>(save_addr);