[lworld] RFR: 8372856: [lworld] link error in release build for oops/test_flatArrayOop.cpp

[Thomas Stuefe]

Simple error with a trivial fix. In release builds, `flatArrayOopDesc::value_at_addr` may be inlined, causing the non-inlined function not to exist and resulting in link errors if a call site does not include the inline hpp.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/valhalla/pull/1765/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1765&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372856
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1765.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1765/head:pull/1765

PR: https://git.openjdk.org/valhalla/pull/1765