RFR: 8334247: [PPC64] Consider trap based nmethod entry barriers

[Martin Doerr]

We can shrink nmethod entry barriers to 4 instructions (from 8) using conditional trap instructions. Performance needs to be evaluated, yet.

-------------

Commit messages:
 - 8334247: [PPC64] Consider trap based nmethod entry barriers

Changes: https://git.openjdk.org/jdk/pull/24135/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24135&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334247
  Stats: 68 lines in 8 files changed: 48 ins; 2 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/24135.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24135/head:pull/24135

PR: https://git.openjdk.org/jdk/pull/24135