RFR: 8290082: [PPC64] ZGC C2 load barrier stub needs to preserve vector registers [v4]

[Martin Doerr]

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

Martin Doerr has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - Update SOE spec for VSR regs. Add comment to register_ppc.hpp
 - Avoid using more than the volatile program storage (288 Bytes) on stack below the SP.
 - 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=03
  Stats: 121 lines in 5 files changed: 41 ins; 6 del; 74 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