[corlib] Use Environment.Platform to access the platform, rather than probing the file system (mono#6535)

Alternatve patch to: mono#6533

Author: migueldeicaza

mcs/class/corlib/System.Runtime.InteropServices.RuntimeInformation/RuntimeInformation.cs

@@ -43,13 +43,12 @@ public static string FrameworkDescription {
 
 		public static bool IsOSPlatform (OSPlatform osPlatform)
 		{
-			switch (Environment.OSVersion.Platform) {
+			switch (Environment.Platform) {
 			case PlatformID.Win32NT:
 				return osPlatform == OSPlatform.Windows;
+			case PlatformID.MacOSX:
+				return osPlatform == OSPlatform.OSX;
 			case PlatformID.Unix:
-				if (File.Exists ("/usr/lib/libc.dylib"))
-					return osPlatform == OSPlatform.OSX;
-
 				return osPlatform == OSPlatform.Linux;
 			default:
 				return false;

mcs/class/corlib/System/Environment.cs

@@ -214,7 +214,7 @@ public static string NewLine {
 		//
 		static OperatingSystem os;
 
-		static extern PlatformID Platform {
+		static internal PlatformID Platform {
 			[MethodImplAttribute (MethodImplOptions.InternalCall)]
 			get;
 		}