RFR: JDK-8303678: [JVMCI] Add possibility to convert object JavaConstant to jobject.

[Tomáš Zezula]

This pull request adds a `jdk.vm.ci.hotspot.HotSpotJVMCIRuntime#getJObjectValue(HotSpotObjectConstant peerObject)` method, which gets a reference to an object in the peer runtime wrapped by the `jdk.vm.ci.hotspot.IndirectHotSpotObjectConstantImpl`. The reference is returned as a HotSpot heap JNI jobject.

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Commit messages:
 - JDK-8303678: Add possibility to convert object JavaConstant to jobject.

Changes: https://git.openjdk.org/jdk/pull/12882/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12882&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303678
  Stats: 20 lines in 1 file changed: 20 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12882.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12882/head:pull/12882

PR: https://git.openjdk.org/jdk/pull/12882