RFR: 8254573: Shenandoah: Streamline/inline native-LRB entry point

[Roman Kennke]

We currently do several calls between the native-LRB entry point and the actual implementation, presumably to resolve
the templated versions. This can be simplified.

Testing: hotspot_gc_shenandoah

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Commit messages:
 - 8254573: Shenandoah: Streamline/inline native-LRB entry point

Changes: https://git.openjdk.java.net/jdk/pull/604/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=604&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254573
  Stats: 63 lines in 3 files changed: 25 ins; 36 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/604.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/604/head:pull/604

PR: https://git.openjdk.java.net/jdk/pull/604