RFR: 8316755: GenShen: Serviceability needs to know about ShenandoahGenerationalHeap

[William Kemper]

Plumb `ShenandoahGenerationalHeap` into `vmStructs`.  

Testing: 

make test TEST=hotspot_serviceability TEST_VM_OPTS="-Xlog:gc*=info:file=/tmp/servicability.log -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahGCMode=generational" CONF=fastdebug

...

Test results: passed: 319; failed: 0

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Commit messages:
 - Add vm struct bindings for ShenandoahGenerationalHeap

Changes: https://git.openjdk.org/shenandoah/pull/329/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=329&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316755
  Stats: 37 lines in 3 files changed: 37 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/329.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/329/head:pull/329

PR: https://git.openjdk.org/shenandoah/pull/329