RFR: 8277926: [aarch64] Address constructors are lacking initialisation.

[Patric Hedlin]

Please review changes to secure proper initialisation in Address constructors.

Changes include (with additional clean-up):

Renamed '**literal**' to '**addr_literal**'.
Renamed '**_ext**' to '**_extend**'.
Renamed Address::mode enum to Address::addr_mode.
Renamed Address::getMode() to Address::mode().
Reorder _mode.

Initialise '**_extend**' to **lsl(0)**.
Initialise '**_target**' to **nullptr**.

Added preconditions to constructors.
Added preconditions to check 'target' address.

Removed '**pcrel**' addressing mode (unused).
Removed direct attribute use in Address.

Minor clean-up to RuntimeAddress use.

---8<---

Testing: tier1-6 (Linux), tier1-3 (MacOSX)

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Commit messages:
 - Cleanup: Renamed Address::mode enum to Address::addr_mode.
 - Cleanup/simplify Address constructors.
 - Reorder _mode. Rename _ext to _extend. Init _target to nullptr.
 - More preconditions. Removed direct attribute use in Address.
 - Removed 'pcrel' addressing mode (unused).
 - Added a few preconditions to check 'target' address.
 - Rename literal to addr_literal.
 - Minor cleanup to RuntimeAddress use.
 - 8277926: [aarch64] Address constructors are lacking initialisation.

Changes: https://git.openjdk.java.net/jdk/pull/7191/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7191&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277926
  Stats: 154 lines in 7 files changed: 36 ins; 12 del; 106 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7191.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7191/head:pull/7191

PR: https://git.openjdk.java.net/jdk/pull/7191