RFR: 8371205: AArch64: Remove unused cmpxchg* methods

[Ruben]

Since JDK-8364406, the AArch64 macroAssembler method cmpxchg_obj_header is no longer used. The method cmpxchgptr is used by cmpxchg_obj_header however is not used by any other method so can be removed alongside cmpxchgptr.
cmpxchgw is also unused and can be removed.

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Commit messages:
 - 8371205: AArch64: Remove unused cmpxchg* methods

Changes: https://git.openjdk.org/jdk/pull/28131/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28131&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371205
  Stats: 101 lines in 2 files changed: 0 ins; 101 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28131.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28131/head:pull/28131

PR: https://git.openjdk.org/jdk/pull/28131