[12] RFR(XS) 8207056: Epsilon GC to support object pinning

[Zhengyu Gu]

Please review this small change to support object pinning in Epsilon GC.

Pinning object in Epsilon GC is no-op, so it is simpler than doing 
GCLock dance.

Bug: https://bugs.openjdk.java.net/browse/JDK-8207056
Webrev: http://cr.openjdk.java.net/~zgu/8207056/webrev.00/

Test:

  gc/epsilon on Linux 64 (fastdebug + release)

Thanks,

-Zhengyu