runtime/internal/syscall: build dummy package if not Linux

Fixes libgo build on non-Linux systems.

Change-Id: I5f0aa08502c5e05b17a504fa6ec2fd9cb713bfb9
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/387134
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

libgo/go/runtime/internal/syscall/syscall_other.go

diff --git a/libgo/go/runtime/internal/syscall/syscall_other.go b/libgo/go/runtime/internal/syscall/syscall_other.go
new file mode 100644
index 0000000..c0945ce
--- /dev/null
+++ b/libgo/go/runtime/internal/syscall/syscall_other.go
@@ -0,0 +1,7 @@
+// Copyright 2022 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.
+
+//go:build !linux
+
+package syscall