RFR: Disable Shenandoah with --with-jvm-features=-shenandoahgc

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/conditional-compilation/webrev.01/

Upstream merge brought us conditional GC compilation. This patch makes sure we are able to build
with Shenandoah disabled. This should allow building Minimal without additional work, and lays the
ground for disabling Shenandoah compilation for unsupported platforms.

Testing: tier1 with -shenandoahgc, tier3_gc_shenandoah with +shenandoahgc

Thanks,
-Aleksey