[jdk17u] RFR: 8276927: [ppc64] Port shenandoahgc to linux on ppc64le

[Martin Doerr]

Backport of 8276927: [PPC64] Port shenandoahgc to linux on ppc64le.
Applies cleanly, but a trivial resolution is required: add additional parameter `, false /*unaligned*/` to `mem2reg` in `ShenandoahBarrierSetAssembler::gen_pre_barrier_stub`.
This is needed because "8269672: C1: Remove unaligned move on all architectures" is not in 17u.

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Commit messages:
 - Backport 57eb864765f38185f8db8f1d37681d6cfe2a3c73

Changes: https://git.openjdk.java.net/jdk17u/pull/287/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=287&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276927
  Stats: 1521 lines in 8 files changed: 1519 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/287.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/287/head:pull/287

PR: https://git.openjdk.java.net/jdk17u/pull/287