RFR 8201785: Make ModRefBarrierSetAssembler abstract on all platforms

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8201785

Webrev:
  http://cr.openjdk.java.net/~shade/8201785/webrev.02/

Found this when while rebasing Epsilon to the recent jdk/jdk. ModRefBSAssembler is abstract on some
platforms (x86, sparc), but not on the others (aarch64, ppc). I think this allows missing the
required overrides of oop_store_at in ModRefBSAssembler subclasses, which should be caught
mechanically. (Epsilon used to defer to ModRefBSAssembler, and it was successfully instantiated!)

PPC folks, would you like to give it a spin on your platforms?

Testing: x86_64 and aarch64 non-PCH builds, Epsilon tests

Thanks,
-Aleksey