[lilliput-jdk17u:lilliput] RFR: 8310742: [Lilliput/JDK17] Revert JVMCI _metadata field removal

[Roman Kennke]

We removed _metadata._klass field from JVMCI interfaces. This is no longer necessary since we introduced UseCompactObjectHeaders, with the caveat that JVMCI can only be used with -UCOH.

Testing:
 - [ ] tier1 -UCOH
 - [ ] tier1 +UCOH

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Commit messages:
 - 8310742: [Lilliput/JDK17] Revert JVMCI _metadata field removal

Changes: https://git.openjdk.org/lilliput-jdk17u/pull/48/files
 Webrev: https://webrevs.openjdk.org/?repo=lilliput-jdk17u&pr=48&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310742
  Stats: 7 lines in 3 files changed: 5 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/lilliput-jdk17u/pull/48.diff
  Fetch: git fetch https://git.openjdk.org/lilliput-jdk17u.git pull/48/head:pull/48

PR: https://git.openjdk.org/lilliput-jdk17u/pull/48