RFR: Fixes needed for mach5

Tue May 31 16:36:23 UTC 2022

On Tue, 31 May 2022 16:21:45 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> This patch adds the wrapper Makefile from the jdk repo to the jextract repo, and moves the existing Makefile contents to `make/Init.gmk` (mirroring jdk).
> 
> The wrapper will set the `topdir` variable, which is then used in `Init.gmk` to include the common makefile, as well as cd into the right working directory before running the `Build.gmk` and `RunTestsPrebuilt.gmk` makefiles

Yeah, the old makefile didn't have a copyright header yet. I can add one

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PR: https://git.openjdk.java.net/jextract/pull/46