RFR: Add 'generational' as value choice for JVM option 'ShenandoahGCMode'.

[Bernd Mathiske]

All upcoming GenShen code will be guarded by this JVM option. If generational mode is not set, Shenandoah will function in its original single-generational form, without changes or degradations.

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Commit messages:
 - Add 'generational' as value choice for JVM option 'ShenandoahGCMode'.

Changes: https://git.openjdk.java.net/shenandoah/pull/4/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=4&range=00
  Stats: 55 lines in 6 files changed: 49 ins; 2 del; 4 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/4.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/4/head:pull/4

PR: https://git.openjdk.java.net/shenandoah/pull/4