Get arm64 to compile.

Change-Id: Ica021aa134388fe7069a5de5df8036ecf0d570f8
diff --git a/libbacktrace/UnwindCurrent.cpp b/libbacktrace/UnwindCurrent.cpp
index 747eb21..d1195ee 100644
--- a/libbacktrace/UnwindCurrent.cpp
+++ b/libbacktrace/UnwindCurrent.cpp
@@ -43,7 +43,7 @@
   #include <asm/sigcontext.h>
   #include <asm/ucontext.h>
   typedef struct ucontext ucontext_t;
-#elif !defined(__mips__)
+#elif !defined(__mips__) && !defined(__aarch64__)
   #error Unsupported architecture.
 #endif