RFR: Conditionally build Shenandoah on supported platforms only

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/conditional-compilation/webrev.02/

Since we have conditional GC compilation in sh/jdk, we can ditch platform-specific stubs on
unsupported platforms, and still remain buildable.

Testing: {x86_64, aarch64, ppc64el, s390x, armhf}, {server, zero} builds

Thanks,
-Aleksey