RFR: Fixes needed for mach5

[Jorn Vernee]

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

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Commit messages:
 - fixes needed for mach5

Changes: https://git.openjdk.java.net/jextract/pull/46/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=46&range=00
  Stats: 117 lines in 2 files changed: 65 ins; 0 del; 52 mod
  Patch: https://git.openjdk.java.net/jextract/pull/46.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/46/head:pull/46

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