RFR(XS): 8221482: Initialize VMRegImpl::regName[] earlier to prevent assert during PrintStubCode

[Schmidt, Lutz]

Dear Community,

may I please request reviews for this tiny change. The purpose is to initialize the regName[] array earlier during VM init. 

Bug:    https://bugs.openjdk.java.net/browse/JDK-8221482 
Webrev: http://cr.openjdk.java.net/~lucy/webrevs/8221482.01/  

Submit-repo tests pending...

Thanks,
Lutz