RFR 8201785: Provide default implementations for ModRefBarrierSetAssembler::oop_store_at for all platforms

[Aleksey Shipilev]

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

Webrev:
  http://cr.openjdk.java.net/~shade/8201785/webrev.01/

Found this when while rebasing Epsilon to the recent jdk/jdk -- Epsilon defers to ModRefBS for
interpreter barriers. It makes more sense to me for ModRefBS to provide the default implementations
for all platforms, rather than it being abstract on some platforms. It follows what AArch64 and PPC
are doing right now, and aligns better with arraycopy barriers already implemented by default in
ModRefBS.

Testing: x86_64 build, Epsilon tests, jdk-submit (running)

Thanks,
-Aleksey