RFR: 8338620: GenShen: Distinguish between young and old collector in GC configuration [v2]

[William Kemper]

> Return `ShenandoahYoung` for `young_collector` under generational mode (continue returning `N/A` for other modes). Return `ShenandoahOld` for `old_collector` under generational mode (continue returning `Shenandoah` for other modes).

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Fix minimal variant build

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/477/files
  - new: https://git.openjdk.org/shenandoah/pull/477/files/3b2e8f45..ecad1ffb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=477&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=477&range=00-01

  Stats: 10 lines in 1 file changed: 6 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/477.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/477/head:pull/477

PR: https://git.openjdk.org/shenandoah/pull/477