pkg/ns: evaluate syscall number at compile-time

Previously this code used a run-time map lookup keyed by
runtime.GOOS/GOARCH.  This version uses conditional compilation to make
this choice at compile time, giving immediate feedback for unsupported
platforms.

pkg/ns/consts_linux_arm.go

@@ -0,0 +1,17 @@
+// Copyright 2015 CNI 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.
+
+package ns
+
+const setNsNr = 375