RFR: Fixes needed for mach5 [v2]

[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

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  add copyright header

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Changes:
  - all: https://git.openjdk.java.net/jextract/pull/46/files
  - new: https://git.openjdk.java.net/jextract/pull/46/files/8b88a006..1b12574f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=46&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=46&range=00-01

  Stats: 25 lines in 1 file changed: 25 ins; 0 del; 0 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