RFR: 8257897: Fix webkit build for XCode 12

[Arun Joseph]

The WebKit build fails with recent Xcode 12.

Bug: `getVTablePointer()` should return a const void*

Fix: Use const void* and remove dependency on JavaVM.framework in Xcode
Upstream fix: https://bugs.webkit.org/show_bug.cgi?id=207871

Test: Build webkit with the Xcode 12 compiler with and without this fix. It should fail without the fix and build with the fix.

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Commit messages:
 - 8257897: Fix webkit build for XCode 12

Changes: https://git.openjdk.java.net/jfx/pull/365/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=365&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257897
  Stats: 8 lines in 3 files changed: 0 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/365.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/365/head:pull/365

PR: https://git.openjdk.java.net/jfx/pull/365