[System.Core] ReaderWriterLockSlim.cs now builds with mono.

* Adds support to build without Code Access Security, by using the
  define DISABLE_CAS_USE.

* Builds without [MethodImpl(MethodImplOptions.AggressiveInlining)]
  when target framework version is lower than 4.5.

* Use Thread.Volatile instead of Volatile.Read when target framework
  version is lower than 4.5.

Required by mono/mono#1436
Without DISABLE_CAS_USE I get the following error:
../../../external/referencesource/System.Core/System/threading/ReaderWriterLockSlim/ReaderWriterLockSlim.cs(60,6): error CS0647: Error during emitting `System.Security.Permissions.HostProtectionAttribute' attribute. The reason is `Object reference not set to an instance of an object'