RFR: 8336256: memcpy short value to int local is incorrect in VtableStubs::unsafe_hash

[Richard Reingruber]

Make sure the local variable used as target of the `memcpy` in `VtableStubs::unsafe_hash` has the same size as `VtableStub::_index` which is the source of the `memcpy`.

With this fix we don't see the crashes reported in the JBS-issue.

JTReg tests: tier1-4 of hotspot and jdk. All of Langtools and jaxp. Renaissance Suite and SAP specific tests.
Testing was done with fastdebug and release builds on the main platforms and also on Linux/PPC64le and AIX.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/20137/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20137&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336256
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20137.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20137/head:pull/20137

PR: https://git.openjdk.org/jdk/pull/20137