RFR: 8307495: Specialize atomic bitset functions for aix-ppc [v2]

[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 two additional commits since the last revision:

 - remove unneeded header guards
 - remove unused var, beautify code

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27650/files
  - new: https://git.openjdk.org/jdk/pull/27650/files/e91818ad..abf0a4f0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27650&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27650&range=00-01

  Stats: 66 lines in 5 files changed: 0 ins; 25 del; 41 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