Adds support for building arm64 Android.

runtime/vm/signal_handler.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_SIGNAL_HANDLER_H_
 #define VM_SIGNAL_HANDLER_H_
 
 #include "vm/allocation.h"
 #include "vm/globals.h"
 
 #if defined(TARGET_OS_LINUX)
 #include <signal.h>  // NOLINT
 #include <ucontext.h>  // NOLINT
 #elif defined(TARGET_OS_ANDROID)
+#include <android/api-level.h>  // NOLINT
+/* Android <= 19 doesn't have ucontext.h */
+#if __ANDROID_API__ <= 19
 #include <signal.h>  // NOLINT
 #include <asm/sigcontext.h>  // NOLINT
 // These are not defined on Android, so we have to define them here.
 typedef struct sigcontext mcontext_t;
 typedef struct ucontext {
   uint32_t uc_flags;
   struct ucontext *uc_link;
   stack_t uc_stack;
   struct sigcontext uc_mcontext;
   uint32_t uc_sigmask;
 } ucontext_t;
+#else
+// Android > 19 has ucontext.h
+#include <signal.h>  // NOLINT
+#include <ucontext.h>  // NOLINT
+#endif  // __ANDROID_API__ <= 19
 #elif defined(TARGET_OS_MACOS)
 #include <signal.h>  // NOLINT
 #include <sys/ucontext.h>  // NOLINT
 #elif defined(TARGET_OS_WINDOWS)
 // Stub out for windows.
 struct siginfo_t;
 struct mcontext_t;
 struct sigset_t {
 };
 #endif
 
 namespace dart {
 
 typedef void (*SignalAction)(int signal, siginfo_t* info,
                              void* context);
 
 class SignalHandler : public AllStatic {
  public:
   static void Install(SignalAction action);
   static uintptr_t GetProgramCounter(const mcontext_t& mcontext);
   static uintptr_t GetFramePointer(const mcontext_t& mcontext);
   static uintptr_t GetStackPointer(const mcontext_t& mcontext);
  private:
 };
 
 
 }  // namespace dart
 
 #endif  // VM_SIGNAL_HANDLER_H_