AArch64: Add arm64 runtime support.

Adds support for arm64 to ART. Assembler stubs are sufficient for
down calls into interpreter. JNI compiler and generics are not finished.

Basic Generic JNI functionality.

Change-Id: I4a07c79d1e037b9f5746673480e32cf456867b82
diff --git a/runtime/arch/context.cc b/runtime/arch/context.cc
index 5eaf809..b1700bb 100644
--- a/runtime/arch/context.cc
+++ b/runtime/arch/context.cc
@@ -18,6 +18,8 @@
 
 #if defined(__arm__)
 #include "arm/context_arm.h"
+#elif defined(__aarch64__)
+#include "arm64/context_arm64.h"
 #elif defined(__mips__)
 #include "mips/context_mips.h"
 #elif defined(__i386__)
@@ -33,6 +35,8 @@
 Context* Context::Create() {
 #if defined(__arm__)
   return new arm::ArmContext();
+#elif defined(__aarch64__)
+  return new arm64::Arm64Context();
 #elif defined(__mips__)
   return new mips::MipsContext();
 #elif defined(__i386__)