RFR(M): 8190430 [MVT] Fixing GC support for Thread-local value buffers

[Frederic Parain]

Please review this changeset fixing the GC support and other
issues with the Thread-Local Value Buffer:

http://cr.openjdk.java.net/~fparain/8190430/webrev.00/

This changeset re-activates the TLVB by default.
The VTBuffer test has been fixed to generate value types
with object references.

All hotspot_valhalla tests pass.

Thank you,

Fred