RFR: 8334247: [PPC64] Consider trap based nmethod entry barriers [v3]

[Martin Doerr]

> We can shrink nmethod entry barriers to 4 instructions (from 8) using conditional trap instructions. Some benchmarks seem to show very small improvements. At least the code size reduction is an advantage.

Martin Doerr has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Move nmethod entry barrier code up in the signal handler.
 - Merge remote-tracking branch 'origin' into 8334247_PPC64_trap_based_nmethod_entry_barrier
 - Merge remote-tracking branch 'origin' into 8334247_PPC64_trap_based_nmethod_entry_barrier
 - 8334247: [PPC64] Consider trap based nmethod entry barriers

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24135/files
  - new: https://git.openjdk.org/jdk/pull/24135/files/dfc6aa5d..16f9d58e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24135&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24135&range=01-02

  Stats: 40720 lines in 1249 files changed: 22188 ins; 12180 del; 6352 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