RFR: 8327000: GenShen: Integrate updated Shenandoah implementation of FreeSet into GenShen [v14]

[Kelvin Nilsen]

> The mainline implementation of ShenandoahFreeSet was recently updated.  This PR integrates the upstream changes
> into Generational Shenandoah.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Resolve delta from openjdk/jdk in external_fragmentation() function

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/440/files
  - new: https://git.openjdk.org/shenandoah/pull/440/files/d113ee73..f7cb541b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=13
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=12-13

  Stats: 8 lines in 1 file changed: 4 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/440.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/440/head:pull/440

PR: https://git.openjdk.org/shenandoah/pull/440