RFR: [11] Make Shenandoah buildable on all platforms

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/alt-platforms-11/webrev.01/

This makes current sh/jdk build without issues on (hopefully) all OpenJDK platforms. It does not add
support on those platforms, but merely enables the builds. I made SPARC and Zero changes blindly.
This is 11-specific patch, backports would need to be fixed separately.

Testing: arm32, ppc64el, s390x cross-builds

Thanks,
-Aleksey