runtime: linux/arm64 cgo support

Change-Id: I309e3df7608b9eef9339196fdc50dedf5f9439f3
Reviewed-on: https://go-review.googlesource.com/8450
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
diff --git a/src/runtime/tls_arm64.h b/src/runtime/tls_arm64.h
new file mode 100644
index 0000000..5416306
--- /dev/null
+++ b/src/runtime/tls_arm64.h
@@ -0,0 +1,14 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#ifdef GOOS_linux
+#define TPIDR TPIDR_EL0
+#define MRS_TPIDR_R0 WORD $0xd53bd040
+#endif
+
+// Define something that will break the build if
+// the GOOS is unknown.
+#ifndef TPIDR
+#define MRS_TPIDR_R0 TPIDR_UNKNOWN
+#endif