RFR: 8257758: Allow building of JavaFX native libs for Apple Silicon

[Johan Vos]

Fix for JDK-8257758

In case the -PtargetArch=arm64 option is supplied to the gradle build, the
native components that are part of the JavaFX build will target this platform.

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Commit messages:
 - Allow to build an arm64-macos build

Changes: https://git.openjdk.java.net/jfx/pull/363/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=363&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257758
  Stats: 10 lines in 1 file changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/363.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/363/head:pull/363

PR: https://git.openjdk.java.net/jfx/pull/363