add arm64 support to goid

Adding a method to get g on Arm64

Signed-off-by: Bin Lu <bin.lu@arm.com>

pkg/goid/BUILD

@@ -7,6 +7,7 @@ go_library(
     srcs = [
         "goid.go",
         "goid_amd64.s",
+        "goid_arm64.s",
         "goid_race.go",
         "goid_unsafe.go",
     ],

pkg/goid/goid_arm64.s

@@ -0,0 +1,21 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "textflag.h"
+
+// func getg() *g
+TEXT ·getg(SB),NOSPLIT,$0-8
+        MOVD g, R0      // g
+        MOVD R0, ret+0(FP)
+        RET