RFR: Revert part of StubGenerator to ease porting

[Roman Kennke]

This reverts our modifications to StubGenerator and related code and
removes the 3rd stub generation phase that used to generate the
Shenandoah stub. This makes it easier to port to platforms, because it
would otherwise require changes in all arches.

The downside is that we cannot use our write-barrier stub in the
interpreter. I put back in a specialized write-barrier for the
interpreter to handle this.

I tested it on x86_64 and aarch64, Aleksey tested it on i586. Both jdk9
and jdk8u.

http://cr.openjdk.java.net/~rkennke/fixstubgen/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/fixstubgen/webrev.00/>

Ok?

Roman