RFR(M): 8207343: Automate vtable/itable stub size calculation

[Schmidt, Lutz]

Dear all, 

may I please request reviews for this change: 

Bug:    https://bugs.openjdk.java.net/browse/JDK-8207343
Webrev: http://cr.openjdk.java.net/~lucy/webrevs/8207343.00/

With this change, I try to get rid of the a-priory size guessing for vtable and itable stubs. Please refer to the bug description for all the details. I didn't want to duplicate that text. 

ARM and AARCH64 help requested!
The edits in vtableStubs_aarch64.cpp and vtableStubs_arm.cpp are made blindfolded. I am neither an ARM expert nor do I have build or test hardware available. I would be very grateful if one of the ARM gurus could please fill in for me.

Thank you!
Lutz