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

* 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
@akoeplinger commit message updated. Thanks!