Adds support for building arm64 Android.

runtime/vm/signal_handler.h

Index: runtime/vm/signal_handler.h
===================================================================
--- runtime/vm/signal_handler.h	(revision 39843)
+++ runtime/vm/signal_handler.h	(working copy)
@@ -12,6 +12,9 @@
 #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.
@@ -23,6 +26,11 @@
   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