Add procid support for arm64 platform

Change-Id: I7c3db8dfdf95a125d7384c1d67c3300dbb99a47e
PiperOrigin-RevId: 223039923

pkg/sentry/platform/procid/BUILD

@@ -7,6 +7,7 @@ go_library(
     srcs = [
         "procid.go",
         "procid_amd64.s",
+        "procid_arm64.s",
     ],
     importpath = "gvisor.googlesource.com/gvisor/pkg/sentry/platform/procid",
     visibility = ["//pkg/sentry:internal"],

pkg/sentry/platform/procid/procid_arm64.s

@@ -0,0 +1,29 @@
+// Copyright 2018 Google LLC
+//
+// 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.
+
+// +build arm64
+// +build go1.8
+// +build !go1.12
+
+#include "textflag.h"
+
+TEXT ·Current(SB),NOSPLIT,$0-8
+	// The offset specified here is the m_procid offset for Go1.8+.
+	// Changes to this offset should be caught by the tests, and major
+	// version changes require an explicit tag change above.
+	MOVD g, R0      // g
+	MOVD 48(R0), R0 // g_m (may change in future versions)
+	MOVD 72(R0), R0 // m_procid (may change in future versions)
+	MOVD R0, ret+0(FP)
+	RET