RFR: 8334247: [PPC64] Consider trap based nmethod entry barriers [v5]

[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 pull request now contains six commits:

 - 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
 - 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: https://git.openjdk.org/jdk/pull/24135/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24135&range=04
  Stats: 67 lines in 8 files changed: 46 ins; 2 del; 19 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