runtime: Add a syscall wrapper package

It allows to support syscall variations between host OSes.

Signed-off-by: Samuel Ortiz <s.ortiz@apple.com>

src/runtime/pkg/syscall/syscall_darwin.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2022 Apple Inc.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+package syscall
+
+import (
+	"syscall"
+)
+
+func Gettid() int {
+	// There is no equivalent to a thread ID on Darwin.
+	// We use the PID instead.
+	return syscall.Getpid()
+}

src/runtime/pkg/syscall/syscall_linux.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2022 Apple Inc.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+
+package syscall
+
+import (
+	"syscall"
+)
+
+func Gettid() int {
+	return syscall.Gettid()
+}