[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.
Merge pull request #2 from esdrubal/readerwriterlockslim

[System.Core] ReaderWriterLockSlim.cs now builds with mono.
Removed unnecessary call to Thread.WriteVolatile.
System.Threading.Volatile is internal on framework versions before 4.5.
As corlib internals are visible from System.Core we can use Volatile.
[System.Core] Disable use of CAS.
The added ifdef is required to circumvent a compile error on older versions of mcs.