Add Conditional Target capabilities to the build scripts

Make Attribute properties immutable

PR Feedback, bugfixes

PR Feedback

scripts/Microsoft.DotNet.Cli.Build.Framework/TargetConditions/BuildArchitecturesAttribute.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+
+namespace Microsoft.DotNet.Cli.Build.Framework
+{
+    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+    public class BuildArchitecturesAttribute : TargetConditionAttribute
+    {
+        private IEnumerable<BuildArchitecture> _buildArchitectures;
+
+        public BuildArchitecturesAttribute(params BuildArchitecture[] architectures)
+        {
+            if (architectures == null)
+            {
+                throw new ArgumentNullException("architectures");
+            }
+
+            _buildArchitectures = architectures;
+        }
+
+        public override bool EvaluateCondition()
+        {
+            var currentArchitecture = CurrentArchitecture.Current;
+
+            if (currentArchitecture == default(BuildArchitecture))
+            {
+                throw new Exception("Unrecognized Architecture");
+            }
+
+            foreach (var architecture in _buildArchitectures)
+            {
+                if (architecture == currentArchitecture)
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+    }
+}