[lworld] RFR: 8375320: [lworld] Revert System.identityHashcode for value objects to non AltSubstitutability version

[Roger Riggs]

Restore the unconditional use of value_object_hash_code_method. 
The Alt hashcode method based on the Unsafe.getFieldMap is not used.
System.identityHashCode uses ValueObjectMethods.valueObjectHashCode() implemented using the original method-handle implementation.

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Commit messages:
 - 8375320: [lworld] Revert SystemIdentityHashcode for wrapper objects non AltSubstitutability version

Changes: https://git.openjdk.org/valhalla/pull/1907/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1907&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375320
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1907.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1907/head:pull/1907

PR: https://git.openjdk.org/valhalla/pull/1907