RFR: 8288181: AArch64: clean up out-of-date comments

[Andrew Haley]

Some of the comments and enum constants in cpu/aarch64 date from the early days of the project and are no longer valid.

-------------

Commit messages:
 - More
 - More
 - 8288181: AArch64: clean up out-of-date comments

Changes: https://git.openjdk.org/jdk/pull/9124/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9124&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288181
  Stats: 18 lines in 7 files changed: 0 ins; 5 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/9124.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9124/head:pull/9124

PR: https://git.openjdk.org/jdk/pull/9124