RFR: Upstream merge to jdk-12+15

[Roman Kennke]

Merge up to jdk-12+15 brings goodies:

- 8212054: Boilerplate to bind oopDesc::equals_raw() to actual raw
implementation
- 8212053: A few more missing object equals barriers
- 8212074: Add method to peek the remaining tasks in task queues
- 8211955: GC abstraction for LAB reserve
- 8212178: Soft reference reclamation race in
com.sun.xml.internal.stream.util.ThreadLocalBufferAllocator

Those are interesting for us too (conc-weakref, conc-coderoots,
conc-classunloading):
- 8210330: Make CLD claiming allow multiple claim bits
- 8210498: nmethod entry barriers

I also included those two:
- 8212616: x86_32 build failures after JDK-8210498 (nmethod entry barriers)
- 8212617: ARM32 build failures after JDK-7041262 (VM_Version should be
called instead of Abstract_VM_Version so that overriding works)


Testing: tier3_gc_shenandoah

Ok?

Roman