Update Interop.PlatformDetection.cs with FreeBSD

It's needed to avoid breaking tests for FreeBSD in one place where we do runtime detection to determine the right value to pass to sysconf to get the current page size.

Author: stephentoub

src/Common/src/Interop/Interop.PlatformDetection.cs

@@ -13,7 +13,8 @@ internal enum OperatingSystem
     {
         Windows,
         Linux,
-        OSX
+        OSX,
+        FreeBSD
     }
 
     internal static bool IsWindows
@@ -31,6 +32,11 @@ internal static bool IsOSX
         get { return OperatingSystem.OSX == PlatformDetection.OperatingSystem; }
     }
 
+    internal static bool IsFreeBSD
+    {
+        get { return OperatingSystem.FreeBSD == PlatformDetection.OperatingSystem; }
+    }
+
     internal static class PlatformDetection
     {
         internal static OperatingSystem OperatingSystem { get { return s_os.Value; } }
@@ -48,6 +54,8 @@ internal static class PlatformDetection
                         {
                             case "Darwin":
                                 return OperatingSystem.OSX;
+                            case "FreeBSD":
+                                return OperatingSystem.FreeBSD;
                             default:
                                 return OperatingSystem.Linux;
                         }