RFR: 8264928: Update to Xcode 12.4

[Kevin Rushforth]

This updates the compiler used to build JavaFX on macOS to Xcode 12.4 (which includes the MacOSX11.1 sdk), which matches the compiler that will be used to build JDK 17, once openjdk/jdk#3388 is integrated.

As noted in the bug report, Xcode 12.4 needs macOS 10.15.4 (Catalina) or later to build. The produced artifacts will be able to run on earlier versions of macOS (for x86_64 systems anyway).

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Commit messages:
 - 8264928: Update to Xcode 12.4

Changes: https://git.openjdk.java.net/jfx/pull/460/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=460&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264928
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/460.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/460/head:pull/460

PR: https://git.openjdk.java.net/jfx/pull/460