RFR: 8200429: Adjust object pinning interface on CollectedHeap

[Per Liden]

The recent additions to CollectedHeap to support object pinning didn't 
quite fit the bill for what the Shenandoah folks had in mind. Instead of 
having CollectedHeap::pin_obejct/unpin_object grab/release the GCLocker 
when object pinning is not supported, we add a 
CollectedHeap::supports_obejct_pinning() and let the caller take the 
appropriate action based on the response.

Bug: https://bugs.openjdk.java.net/browse/JDK-8200429
Webrev: http://cr.openjdk.java.net/~pliden/8200429/webrev.0

/Per