RFR(XS): 8155946: Minimal VM fails to built after 8154153: PS: Restore preserved marks in parallel

[Leonid Mesnik]

Hi

Please review following small fix which allows to build minimal VM.
http://cr.openjdk.java.net/~lmesnik/8155946/webrev.00/ 
<http://cr.openjdk.java.net/%7Elmesnik/8155946/webrev.00/>

This fix just guarded parallel GC specific code in vm/gc/shared. The 
full fix would be moving parallel specific code into vm/gc/parallel.
I am going to file separate issue for it.

Testing JPRT which minimal/client/server builds with tests.

Leonid