RFR: 8265469: Allow to build media and webkit for Linux-AArch64

[Johan Vos]

Changes that allow to build linux configuration on Linux AArch64
This PR introduces an `IS_AARCH64` parameter in build.gradle.
This PR already contains the change from PR #465 so if this one gets integrated, that change needs to removed from this PR.

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Commit messages:
 - Changes that allow to build linux configuration on Linux AArch64

Changes: https://git.openjdk.java.net/jfx/pull/467/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=467&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265469
  Stats: 33 lines in 4 files changed: 20 ins; 3 del; 10 mod
  Patch: https://git.openjdk.java.net/jfx/pull/467.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/467/head:pull/467

PR: https://git.openjdk.java.net/jfx/pull/467