Address review comments

- Add license header to fpunwind_arm64.s
- Add comment explaining the motivation for TestSetNoClobberFramePointer

arrow/internal/testing/tools/fpunwind_arm64.s

@@ -1,3 +1,19 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you 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.
+
 TEXT ·FPUnwind(SB), $0-0
 	MOVD R29, R19
 loop:

arrow/memory/memory_test.go

@@ -129,6 +129,7 @@ func TestSetNoClobberFramePointer(t *testing.T) {
 	var b [8]byte
 	memory.Set(b[:], 0xff)
 	// We should be able to safely frame pointer unwind after calling this
-	// function
+	// function. The arm64 assembly implementation used to clobber the
+	// frame pointer. See #150
 	tools.FPUnwind()
 }