RFR: 8296349: [aarch64] Avoid slicing Address::extend

[Kim Barrett]

Please review this change around `Address::extend`.  The 4 derived classes are
replaced by static functions of the same name as the former class.  These
functions return an `extend` object initialized with the same values as were
used by the corresponding derived class constructor.

Testing: mach5 tier1-3

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Commit messages:
 - flatten Address::extend

Changes: https://git.openjdk.org/jdk/pull/10976/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10976&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296349
  Stats: 20 lines in 2 files changed: 0 ins; 14 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/10976.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10976/head:pull/10976

PR: https://git.openjdk.org/jdk/pull/10976