RFR: 8290082: [PPC64] ZGC C2 load barrier stub needs to preserve vector registers [v2]

[Martin Doerr]

> Preserve volatile vector registers in ZGC C2 load barrier stub.

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Avoid using more than the volatile program storage (288 Bytes) on stack below the SP.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9453/files
  - new: https://git.openjdk.org/jdk/pull/9453/files/a1ca2ea5..bb0513c1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9453&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9453&range=00-01

  Stats: 29 lines in 1 file changed: 11 ins; 5 del; 13 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