RFR: 8290082: [PPC64] ZGC C2 load barrier stub needs to preserve vector registers

[Martin Doerr]

Preserve volatile vector registers in ZGC C2 load barrier stub.

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Commit messages:
 - 8290082: [PPC64] ZGC C2 load barrier stub needs to preserve vector registers

Changes: https://git.openjdk.org/jdk/pull/9453/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9453&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290082
  Stats: 72 lines in 4 files changed: 27 ins; 2 del; 43 mod
  Patch: https://git.openjdk.org/jdk/pull/9453.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9453/head:pull/9453

PR: https://git.openjdk.org/jdk/pull/9453