RFR: Add support for Linux/AArch64

[Jorn Vernee]

Currently we don't support jextract on Linux/AArch64, but have had some request to add support. This PR contains changes to make that work.

I've added a build script for the libclang bundle, which is loosely based on the old build script we had in the JDK repo, that can be used to create a libclang bundle. (and which I did in fact use to create the libclang bundle for aarch64). I've only tested this script on Linux/AArch64, but have included configurations for mac/linux x64/aarch64 as well.

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Commit messages:
 - DEVKIT_HOME -> DEVKIT_DIR
 - Drop executable permission
 - Tweak build script
 - Add some more documentation to build script
 - fix gradle build
 - fix failing test
 - Fix build script
 - Add libclang build script
 - Add aarch64 as make target platform

Changes: https://git.openjdk.org/jextract/pull/258/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=258&range=00
  Stats: 176 lines in 5 files changed: 173 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jextract/pull/258.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/258/head:pull/258

PR: https://git.openjdk.org/jextract/pull/258