RFR: 8307495: Specialize atomic bitset functions for aix-ppc [v5]

[David Briemann]

> Add atomic bitset functions for PPC64.
> Refactor so that inline assembler code is shared for all PPC64 platforms.
> Add back a missing "simple guard" to PlatformCpmxchg<1>. Remove some dead Power7 code.

David Briemann has updated the pull request incrementally with one additional commit since the last revision:

  beautify add_then_fetch

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27650/files
  - new: https://git.openjdk.org/jdk/pull/27650/files/a1252228..9aa64ff9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27650&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27650&range=03-04

  Stats: 14 lines in 1 file changed: 2 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/27650.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27650/head:pull/27650

PR: https://git.openjdk.org/jdk/pull/27650