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

Fixes libgo build on non-Linux systems.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/387134

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-20e74f9ef8206fb02fd28ce3d6e0f01f6fb95dc9
+aee8eddbfc3ef1b03353a060e79e7d668fb229e2
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

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