RFR: 8283626: AArch64: Set relocInfo::offset_unit to 4

[Evgeny Astigeevich]

AArch64 instructions size is 4 bytes. Two lowest bits of `relocInfo` offsets can be discarded.
Tested with release and fastdebug builds:
- `gtest`: Passed.
- `tier1`...`tier4`: Passed

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Commit messages:
 - 8283626: AArch64: Set relocInfo::offset_unit to 4

Changes: https://git.openjdk.java.net/jdk/pull/7991/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7991&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283626
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7991.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7991/head:pull/7991

PR: https://git.openjdk.java.net/jdk/pull/7991