RFR: 8251858: Update to Xcode 11.3.1

[Kevin Rushforth]

This updates the compiler used to build JavaFX on macOS to Xcode 11.3.1 + MacOSX10.15 sdk, which matches the compiler
used to build JDK 16.

As noted in the bug report, Xcode 11.3.1 needs macOS 10.14.4 (Mojave) or later to build, but the produced artifacts
will be able to run on earlier versions of macOS.

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Commit messages:
 - 8251858: Update to Xcode 11.3.1

Changes: https://git.openjdk.java.net/jfx/pull/291/files
 Webrev: https://webrevs.openjdk.java.net/jfx/291/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251858
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/291.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/291/head:pull/291

PR: https://git.openjdk.java.net/jfx/pull/291