RFR 8245289: Clean up offset code in JavaClasses

[coleen.phillimore at oracle.com]

Summary: Make offset member names consistent and private, move static 
initializations near owning classes

This one is better.  I gave up on the X macros because they didn't save 
typing.  The changes here were to add underscores to offset field names, 
remove _in_bytes for offset accessors and add asserts that the offset is 
non-zero, made java_lang_ref_Reference offset fields private, and moved 
static member definitions closer to the class instead of some random 
place in javaClasses.cpp.

See discussion for 8243996 Remove hardcoded field offsets for more 
details. 
https://mail.openjdk.java.net/pipermail/hotspot-dev/2020-May/041732.html

Tested with tier1-6.

open webrev at http://cr.openjdk.java.net/~coleenp/2020/8245289.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8245289

Thanks,
Coleen