RFR: 8274024: Use regular accessors to internal fields of oopDesc

[Roman Kennke]

Currently, we are using 'raw' accessors to initialize the mark, Klass*, (array-)length and klass_gap of oops. This is ugly and confusing and we should just use the regular accessors.

Testing:
 - [ ] tier1
 - [ ] tier2
 - [ ] hotspot_gc

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Commit messages:
 - 8274024: Use regular accessors to internal fields of oopDesc

Changes: https://git.openjdk.java.net/jdk/pull/5585/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5585&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274024
  Stats: 37 lines in 5 files changed: 1 ins; 17 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5585.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5585/head:pull/5585

PR: https://git.openjdk.java.net/jdk/pull/5585