[lworld] RFR: 8370714: [lworld] Reimplement System.identityHashCode for Value Objects

[Roger Riggs]

The alternate hashcode compution is enabled when the alternate subsitutability method is enabled with: `-XX:+UseAltSubstitutabilityMethod`. 
The IdentityHashCode is computed using the map of field offsets and sizes from Unsafe. 
The basis of hashCode is the the hashCode of the object's class mixing in the value of each primitive field and the hashCode of each reference.

The changes to Hotspot invoke the original or the alternate Java method based on the VM `UseAltSubstitutabilityMethod` command line flag.

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Commit messages:
 - 8370714: [lworld] Reimplement System.identityHashCode for Value Objects

Changes: https://git.openjdk.org/valhalla/pull/1773/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1773&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370714
  Stats: 85 lines in 7 files changed: 71 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/valhalla/pull/1773.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1773/head:pull/1773

PR: https://git.openjdk.org/valhalla/pull/1773