RFR: 8287433: [PPC64] g1_write_barrier_pre needs extension for Loom

[Martin Doerr]

Loom uses `c2i_entry_barrier` which needs to preserve more registers than currently implemented `g1_write_barrier_pre`. See JBS for details.

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Commit messages:
 - 8287433: [PPC64] g1_write_barrier_pre needs extension for Loom

Changes: https://git.openjdk.java.net/jdk/pull/8918/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8918&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287433
  Stats: 21 lines in 1 file changed: 16 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8918.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8918/head:pull/8918

PR: https://git.openjdk.java.net/jdk/pull/8918